Fix shards db name typo from previous commit
Fix shards db name typo from previous commit

Previous commit which switched to using mem3 for replicator shard
discovery introduced a typo.

 `config:get("mem3", "shard_db", "dbs")`

should be:

 `config:get("mem3", "shards_db", "_dbs")`

COUCHDB-3277

fkaempfer/couchdb,fkaempfer/couchdb,fkaempfer/couchdb,fkaempfer/couchdb,fkaempfer/couchdb,fkaempfer/couchdb