Summary:
Update the TDigest interface to accept unsorted or sorted data.
If unsorted, use a custom radix sort explicitly aimed at this use case:
doubles only, assume small data, so we don't use in-place and everything still
fits in cache (1000-10000 items).

Previously D8875766.  This version does not require newer boost versions, and puts
everything in the heap, so there should be no fiber overflows.

Reviewed By: yfeldblum

Differential Revision: D9197722

fbshipit-source-id: 10088cf14d1ff88e1072c00084e09129271e97ec