Summary:
In Mononoke for a sharded DB we historically used connection pool size 1 per shard. With the Mysql FFI client now it doesn't make sense, as the client's conn pool is smart enough and designed to works with sharded DBs, so currently we don't even benefit from having a pool.
In this diff I added an API to create sharded connections: a single pool is shared between all the shards.
Reviewed By: farnz
Differential Revision: D24475317
fbshipit-source-id: b7142c030a10ccfde1d5a44943b38cfa70332c6a