jemalloc: Implement release_free_memory

Implement the 'release_free_memoy' alloc hook for jemalloc. This
purges all dirty (free, but still mapped in the process) pages.

Change-Id: I73b98480a984b0659c0e655b0bd6f6a3125accc1
Reviewed-on: http://review.couchbase.org/41485
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>