all: dump trie journal into file

[jsvisa]

After the bloating, the size of trie journal will reached into 4GB++, will exceed pebble's write bactch size(4GB), so after the geth stop, it will panic as below:

INFO [06-04|00:49:21.888] HTTP server stopped                      endpoint=[::]:8543
INFO [06-04|00:49:21.889] IPC endpoint closed                      url=/data/geth-dev/geth.ipc
INFO [06-04|00:49:21.889] Ethereum protocol stopped
INFO [06-04|00:49:21.889] Transaction pool stopped
INFO [06-04|00:49:23.669] Imported new potential chain segment     number=64 hash=745cbd..6e876d blocks=1 txs=0 mgas=0.000 elapsed=2.055s     mgasps=0.000 triediffs=5.89GiB    triedirty=0.00B
INFO [06-04|00:49:23.670] Persisting dirty state to disk           head=63 root=6ac6c8..71f7de layers=63
WARN [06-04|00:49:23.670] Error performing sealing work            err="blockchain is stopped"
panic: pebble: batch too large: >= 4.0GB

goroutine 1 [running]:
github.com/cockroachdb/pebble.(*Batch).grow(0xc06e685200?, 0xc000138908?)
        github.com/cockroachdb/pebble@v1.1.5/batch.go:1414 +0x12d
github.com/cockroachdb/pebble.(*Batch).prepareDeferredKeyValueRecord(0xc06e685200, 0xb, 0x120482415, 0x1)
        github.com/cockroachdb/pebble@v1.1.5/batch.go:592 +0x93
github.com/cockroachdb/pebble.(*Batch).SetDeferred(...)
        github.com/cockroachdb/pebble@v1.1.5/batch.go:726
github.com/cockroachdb/pebble.(*Batch).Set(0xc06e685200, {0x2f91bf0, 0xb, 0x2712?}, {0xc7c96b4000, 0x120482415, 0x80?}, 0x80?)
        github.com/cockroachdb/pebble@v1.1.5/batch.go:708 +0x3c
github.com/cockroachdb/pebble.(*DB).Set(0xc0005c1b08, {0x2f91bf0, 0xb, 0xb}, {0xc7c96b4000, 0x120482415, 0x200000000}, 0x319ecd4)
        github.com/cockroachdb/pebble@v1.1.5/db.go:608 +0xd2
github.com/ethereum/go-ethereum/ethdb/pebble.(*Database).Put(0x20?, {0x2f91bf0?, 0x7f1bb2504a38?, 0x217c148?}, {0xc7c96b4000?, 0x6e7340?, 0x319f7a0?})
        github.com/ethereum/go-ethereum/ethdb/pebble/pebble.go:388 +0x14c
github.com/ethereum/go-ethereum/core/rawdb.WriteTrieJournal({0x7f1b484a0940?, 0xc000686660?}, {0xc7c96b4000?, 0xc404fcba20?, 0xc30cf0ea80?})
        github.com/ethereum/go-ethereum/core/rawdb/accessors_state.go:154 +0x52
github.com/ethereum/go-ethereum/triedb/pathdb.(*Database).Journal(0xc00107bda0, {0x6a, 0xc6, 0xc8, 0x52, 0x51, 0x42, 0xd4, 0x8d, 0x2c, ...})
        github.com/ethereum/go-ethereum/triedb/pathdb/journal.go:337 +0x705
github.com/ethereum/go-ethereum/triedb.(*Database).Journal(0xc000878e10?, {0x6a, 0xc6, 0xc8, 0x52, 0x51, 0x42, 0xd4, 0x8d, 0x2c, ...})
        github.com/ethereum/go-ethereum/triedb/database.go:312 +0x3a
github.com/ethereum/go-ethereum/core.(*BlockChain).Stop(0xc0412eac08)
        github.com/ethereum/go-ethereum/core/blockchain.go:1237 +0x225
github.com/ethereum/go-ethereum/eth.(*Ethereum).Stop(0xc000530340)
        github.com/ethereum/go-ethereum/eth/backend.go:517 +0xc5
github.com/ethereum/go-ethereum/node.(*Node).stopServices(0xc0001b49a0, {0xc0413eb380, 0x3, 0x1000000020002?})
        github.com/ethereum/go-ethereum/node/node.go:291 +0xbf
github.com/ethereum/go-ethereum/node.(*Node).Close(0xc0001b49a0)

Here we try to save the journal into file to bypass this issue.

[gballet]

I'd prefer to store it as multiple keys into the db, but the batch size problem will probably also affect the full batch. Let's not make this a blocker for the bloatnet.