Properly maintain tcache_bin_t's avail pointer such that it is NULL if
no objects are cached. This only caused problems during thread cache
destruction, since cache flushing otherwise never occurs on an empty
bin.
Remove medium size classes, because concurrent dirty page purging is
no longer capable of purging inactive dirty pages inside active runs
(due to recent arena/bin locking changes).
Enhance tcache to support caching large objects, so that the same range
of size classes is still cached, despite the removal of medium size
class support.
Initialize small run header before dropping arena->lock,
arena_chunk_purge() relies on valid small run headers during run
iteration.
Add some assertions.
For bin-related allocation, protect data structures with bin locks
rather than arena locks. Arena locks remain for run
allocation/deallocation and other miscellaneous operations.
Restructure statistics counters to maintain per bin
allocated/nmalloc/ndalloc, but continue to provide arena-wide statistics
via aggregation in the ctl code.
Use chains of cached objects, rather than using arrays of pointers.
Since tcache_bin_t is no longer dynamically sized, convert tcache_t's
tbin to an array of structures, rather than an array of pointers. This
implicitly removes tcache_bin_{create,destroy}(), which further
simplifies the fast path for malloc/free.
Use cacheline alignment for tcache_t allocations.
Remove runtime configuration option for number of tcache bin slots, and
replace it with a boolean option for enabling/disabling tcache.
Limit the number of tcache objects to the lesser of TCACHE_NSLOTS_MAX
and 2X the number of regions per run for the size class.
For GC-triggered flush, discard 3/4 of the objects below the low water
mark, rather than 1/2.
