Disable top chunk split optimization for hashed sharding

Using a hashed shard key, we should not be doing the 100/0 split on the maxKey range. Doing so means that we split it unevenly and thus its turn to split will happen again on the larger portion of it, and then when maxKey chunk fills again, it will again be split "unevenly".

Since a lot of the times balancing will be disabled with hashed shard key (since gross uneven distribution of chunks is not likely) you may end up with shard having maxKey chunk with very large number of chunks (though generally the same number of overall documents).