The current method of segregating partitions doesn't work well for huge
number of small partitions. For especially bad input, it can produce
hundreds or even thousands of buckets. This patch adds a new segregator
specialized for this use-case. This segregator uses a memtable to sort
out-of-order partitions in-memory. When the memtable size reaches the
provided max-memory limit, it is flushed to disk and a new empty one is
created. In-order partitions bypass the sorting altogether and go to the
fast-path bucket.
The new method is not used yet, this will come in the next patch.
18599f26fa