threads: reimplement tid allocation on top of a bitmap

There are workloads with very bursty tid allocation and since unr tries very
hard to have small-sized bitmaps it keeps reallocating memory. Just doing
buildkernel gives almost 150k calls to free coming from unr.

This also gets rid of the hack which tried to postpone TID reuse.

Reviewed by: kib, markj
Tested by: pho
Differential Revision: https://reviews.freebsd.org/D27101