Prefix leveldb debug logging
Add leveldb: prefix to leveldb debug logging lines.
leveldb debug messages come in various scary flavors such as:
2017-11-30 08:26:31 leveldb: Recovering log #26
2017-11-30 08:26:31 leveldb: Level-0 table #28: started
2017-11-30 08:26:31 leveldb: Level-0 table #28: 597 bytes OK
2017-11-30 08:26:31 leveldb: Delete type=0 #26
2017-11-30 08:26:31 leveldb: Delete type=3 #24
so it's reasonably important to mark them as coming from leveldb
internals and not from consensus validation wallet or such.
(this only affects `-debug=leveldb` or `-debug=1` otherwise you won't
see them in the first place)