Allow mem3_shards:local to take a list or binary

BugzId: 29571
Get the shard suffix for a given database

This grabs the shards for the given database name, and then pulls out
the first shard and extracts out the suffix. mem3:shards is ets
backed, so in the general case this should be fast.

BugzId: 29571
Merge pull request #70 from cloudant/29571-metadatadb

29571 metadatadb