Support OP_MSG exhaust cursors on mongos

SERVER-36105 added support for OP_MSG exhaust cursors for both mongod and mongos in 4.2. Later in SERVER-44517 support for exhaust on mongos was removed in 4.4 (for the good reason that the mongos code path was untested). This ticket request that OP_MSG exhaust cursors be officially supported on mongos.

DRIVERS-535 depends on this feature since we don't want to add a feature to drivers which doesn't work in sharded clusters. The motivation is that exhaust cursors can provide better query performance.

Another potential improvement is for Load Balanced clusters. When the driver is connected through a load balancer it automatically pins the connection to the cursor. Since the connection is already pinned, it would be possible to automatically use exhaust cursors to (potentially) improve performance.