A shard may be removed by mongos, but open cursors may remain on the final migration's data waiting to be deleted. So long as these cursors are open, the data will not be deleted and the shard cannot be re-added. Need to also verify that all databases will be removed in this case.

We should probably check for this case explicitly and check that the shard's sharded databases are removed before removing the shard. (Say that 5 times fast)