It is possible for some replicas of a container DB to be sharded, and
for the container to then be emptied and deleted, before another
replica of the DB has started sharding. Previously, the unsharded
replica would remain in the unsharded state because the sharder would
not process deleted DBs.
This patch modifies the sharder to always process deleted DBs; this
will result in the sharder making some additional DB queries for shard
ranges in order to determine whether any processing is required for
deleted DBs.
Auto-sharding will not act on deleted DBs.
Change-Id: Ia6ad92042aa9a51e3ddefec0b9b8acae44e9e9d7