Rename shards to pool

Shard turned out to be a quite overloaded/confusing term to use here,
although correct. We decided to rename our shard feature to pool.

P.S: I swear I didn't use sed O.O

Change-Id: Ic54f29a4da7d7690c9c9210b74876b96f0ae0eac
Merge "Rename shards to pool"