Move checkShardingIndex outside shardCollection's read critical section on the primary shard

The shardCollection implementation on the primary shard contains at least one task, which performs collection scan while under the critical section, namely checkShardingIndex. This can cause a significant period of unavailability for a large collection.

This ticket is to inspect all the work which is performed under the critical section of shardCollection and ensure that it is held for the minimum time possible.