9b80fc1 Prefix leveldb debug logging (Wladimir J. van der Laan)

Pull request description:

  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 is consistent with the `libevent:` prefix for libevent messages.

  (this only affects `-debug=leveldb` or `-debug=1` otherwise you won't see them in the first place)

Tree-SHA512: 074eba00d39c6378b9e14d03aa2c551b4e3fc7bffdd5a1a0ba2498b44bcc77071d47735be09493286056053f0cca625c5cbaad7ad1ddb9d902d30e7cd316d9d2