support multiple database backends

rocksdb does not build reliable.

changed milestone to %v0.1.0: production ready

assigned to @paul

redis oder memcached?

added status: Doing label

assigned to @yksflip and unassigned @paul

mentioned in merge request !2

changed title from switch KV database to something else to support multiple database backends

removed milestone %v0.1.0: production ready

removed status: Doing label