libs/db: close boltDBIterator (#3627)
libs/db: close boltDBIterator (#3627)

Refs https://github.com/tendermint/tendermint/pull/3610#discussion_r281201274

If we do not close, other txs will be stuck forever.

tendermint/tendermint,tendermint/tendermint,tendermint/tendermint,tendermint/tendermint