Clean up warnings in version3 and master builds.

[evoskuil]

libbitcoin-system is clean, with the exception of 4 unused function warnings by 2 older compilers, which are bogus.

vc++141 is clean, but vc140 is not and vc120 has a break resulting from a dependency update (boost?).

[evoskuil]

libsecp256k1:
https://travis-ci.org/github/libbitcoin/libbitcoin-database/jobs/763518509#L626
./src/secp256k1.c:79:9: warning: missing field 'pre_g_128' initializer
https://travis-ci.org/github/libbitcoin/libbitcoin-database/jobs/763518509#L630
./src/secp256k1.c:80:9: warning: missing field 'blind' initializer


https://travis-ci.org/github/libbitcoin/libbitcoin-consensus/jobs/763517820#L799
./src/clone/serialize.h:632:6: warning: extra ‘;’ [-Wpedantic]
https://travis-ci.org/github/libbitcoin/libbitcoin-consensus/jobs/763517820#L814
src/clone/crypto/sha256.cpp:465:6: warning: ‘bool {anonymous}::SelfTest()’ defined but not used [-Wunused-function]
assert(SelfTest());
If NDEBUG is not defined, then assert checks if its argument (which must have scalar type) compares equal to zero.
https://travis-ci.org/github/libbitcoin/libbitcoin-consensus/jobs/763517820#L832
src/clone/script/interpreter.cpp:429:1: warning: control reaches end of non-void function [-Wreturn-type]
{
    switch (sigversion) {
    case SigVersion::BASE:
    case SigVersion::WITNESS_V0:
        return EvalChecksigPreTapscript(sig, pubkey, pbegincodehash, pend, flags, checker, sigversion, serror, success);
    case SigVersion::TAPSCRIPT:
        return EvalChecksigTapscript(sig, pubkey, execdata, flags, checker, sigversion, serror, success);
    case SigVersion::TAPROOT:
        // Key path spending in Taproot has no script, so this is unreachable.
        break;
    }
    assert(false);
}
https://travis-ci.org/github/libbitcoin/libbitcoin-consensus/jobs/763517820#L837
src/clone/script/interpreter.cpp:1542:69: warning: ‘ext_flag’ may be used uninitialized in this function [-Wmaybe-uninitialized]
    uint8_t ext_flag, key_version;
    switch (sigversion) {
    case SigVersion::TAPROOT:
        ext_flag = 0;
        // key_version is not used and left uninitialized.
        break;
    case SigVersion::TAPSCRIPT:
        ext_flag = 1;
        // key_version must be 0 for now, representing the current version of
        // 32-byte public keys in the tapscript signature opcode execution.
        // An upgradable public key version (with a size not 32-byte) may
        // request a different key_version with a new sigversion.
        key_version = 0;
        break;
    default:
        assert(false);
    }


https://travis-ci.org/github/libbitcoin/libbitcoin-database/jobs/763518508#L1503
/usr/lib/gcc/x86_64-linux-gnu/5.4.0/../../../../include/c++/5.4.0/ext/new_allocator.h:124:29: warning: 
destructor called on non-final 'libbitcoin::database::accessor' that has
virtual functions but non-virtual destructor [-Wdelete-non-virtual-dtor]
https://travis-ci.org/github/libbitcoin/libbitcoin-database/jobs/763518508#L1309
src/databases/filter_database.cpp:43:23: warning: unused variable
'filter_header_size' [-Wunused-const-variable]
static constexpr auto filter_header_size = hash_size;
https://travis-ci.org/github/libbitcoin/libbitcoin-database/jobs/763518508#L1464
test/databases/block_database.cpp:362:11: warning: unused variable 'header0'
[-Wunused-variable]


https://travis-ci.org/github/libbitcoin/libbitcoin-blockchain/jobs/763518687#L1926
./include/bitcoin/blockchain/pools/transaction_entry.hpp:135:14: warning: 
private field 'locktime_' is not used [-Wunused-private-field]
https://travis-ci.org/github/libbitcoin/libbitcoin-blockchain/jobs/763518687#L1930
./include/bitcoin/blockchain/pools/transaction_entry.hpp:136:14: warning: 
private field 'min_spendable_height_' is not used [-Wunused-private-field]
https://travis-ci.org/github/libbitcoin/libbitcoin-blockchain/jobs/763518687#L1954
./include/bitcoin/blockchain/validate/validate_transaction.hpp:62:16: warning: 
private field 'retarget_' is not used [-Wunused-private-field]


https://travis-ci.org/github/libbitcoin/libbitcoin-node/jobs/763518896#L2436
src/full_node.cpp:281:17: warning: unused variable 'times' [-Wunused-variable]
https://travis-ci.org/github/libbitcoin/libbitcoin-node/jobs/763518896#L2440
./include/bitcoin/node/full_node.hpp:136:20: warning: private field
'protocol_maximum_' is not used [-Wunused-private-field]
https://travis-ci.org/github/libbitcoin/libbitcoin-node/jobs/763518896#L2452
./include/bitcoin/node/protocols/protocol_compact_filter_out.hpp:73:16: warning: 
private field 'node_' is not used [-Wunused-private-field]
https://travis-ci.org/github/libbitcoin/libbitcoin-node/jobs/763518896#L2468
./include/bitcoin/node/utility/reservation.hpp:152:17: warning: private field
'maximum_deviation_' is not used [-Wunused-private-field]
https://travis-ci.org/github/libbitcoin/libbitcoin-node/jobs/763518896#L2475
console/executor.cpp:49:19: warning: unused variable 'mode' [-Wunused-const-variable]


One for every command:
https://travis-ci.org/github/libbitcoin/libbitcoin-explorer/jobs/763519126#L2516
/usr/lib/gcc/x86_64-linux-gnu/5.4.0/../../../../include/c++/5.4.0/ext/new_allocator.h:124:29: warning: 
destructor called on non-final
'libbitcoin::explorer::commands::address_decode' that has virtual
functions but non-virtual destructor [-Wdelete-non-virtual-dtor]


https://travis-ci.org/github/libbitcoin/libbitcoin-server/jobs/763519248#L3442
src/interface/blockchain.cpp:42:25: warning: unused variable 'index_size'
https://travis-ci.org/github/libbitcoin/libbitcoin-server/jobs/763519248#L3466
./include/bitcoin/server/workers/notification_worker.hpp:128:16: warning: 
private field 'secure_' is not used [-Wunused-private-field]
https://travis-ci.org/github/libbitcoin/libbitcoin-server/jobs/763519248#L3473
console/executor.cpp:51:19: warning: unused variable 'mode' [-Wunused-const-variable]