Add the ability to use any Tendermint DB backend

[RiccardoM]

Summary

Allow the users to use any Tendermint DB backend while creating Cosmos-specific databases.

Problem Definition

Currently when creating a new LevelDB, the only two supported database backends are LevelDB and ClevelDB. While this works great for most of the chains, someone might want to use different backends such as the ones supported by Tenderming (e.g. BoltDB or RocksDB).

Proposal

Implement such option would be a matter of changing the following lines inside the types/utils.go file:

if DBBackend == string(dbm.CLevelDBBackend) {
	backend = dbm.CLevelDBBackend
}

to become

if len(DBBackend) != 0 {
	backend = dbm.BackendType(DBBackend)
}

This change would be 100% compatible with the current code and allows for further customization while compiling the binaries.

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