Support some other storage backends

[ebrahim]

There are a few other interesting technologies to use as storage backend which may provide better performance in contrast to BDB:

• Kyoto Cabinet
• Tokyo Cabinet
• LevelDB

[oza]

Thank you for your comment.
In my anticipation, LevelDB seems to be fit for our cases because accord accesses backend DB from single thread.

I'll benchmark it, and set the best DB to the default backend DB for accord.

[skull-squadron]

Noticed Riak 1.x added support eLevelDB sometime ago.

And for use-cases less like Flume and more like write-heavy ZK, BitCask might be a good choice.