MX-1232, MX-1139, implement limited sharding of partitioned collections:
 - create collection via shardCollection
 - disable splitVector function for sharded+partitioned collections
 - add an option to dropPartition to accept a pivot instead of a partitionID
 - implement addPartition, getPartitionInfo, and dropPartition through mongos
 - clone partition info when a migrate creates a collection on a shard