* Fix a race in chunk_dealloc_dss().
* Check for allocation failure before zeroing memory in base_calloc().
Merge enhancements from a divergent version of jemalloc:
* Convert thread-specific caching from magazines to an algorithm that is
more tunable, and implement incremental GC.
* Add support for medium size classes, [4KiB..32KiB], 2KiB apart by
default.
* Add dirty page tracking for pages within active small/medium object
runs. This allows malloc to track precisely which pages are in active
use, which makes dirty page purging more effective.
* Base maximum dirty page count on proportion of active memory.
* Use optional zeroing in arena_chunk_alloc() to avoid needless zeroing
of chunks. This is useful in the context of DSS allocation, since a
long-lived application may commonly recycle chunks.
* Increase the default chunk size from 1MiB to 4MiB.
Remove feature:
* Remove the dynamic rebalancing code, since thread caching reduces its
utility.
