Split shards by a route key

Provide a way to split a shard using a route key such that all documents of the specified route key end up in a single dedicated sub-shard.

Example:
Assume that collection1, shard1 has hash range [0, 20]. Also that route key 'A!' has hash range [12,15]. Then invoking:

/admin/collections?action=SPLIT&collection=collection1&split.key=A!

should produce three sub-shards with hash range [0,11], [12,15] and [16,20].

Specifying the source shard is not required here because the route key is enough to figure it out. Route keys spanning more than one shards will not be supported.

Note that the sub-shard with the hash range of the route key may also contain documents for other route keys whose hash ranges overlap.