Split handler enforces the constraint that a collision bucket is never split
across a shard boundary. This is a necessary constraint. If we were to permit
the collision bucket to be split across a shard boundary, then the scan for a
matching entry in the BLOBS index will no longer be shard local and therefore
would no longer be ACID (that is, it will be busted). The constraint is
enforced by requiring the separator key to create a distinction in the prefix
bytes which exclude the collision counter.