libs/db: boltdb: use slice instead of sync.Map (#3633)
libs/db: boltdb: use slice instead of sync.Map (#3633)

for storing batch keys and values in boltDBBatch.

NOTE: batch does not have to be safe for concurrent access. Delete may
be slow, but given it should not be used often, it's ok.

Fixes #3631
tendermint/tendermint,tendermint/tendermint,tendermint/tendermint,tendermint/tendermint