MB-12067 [jemalloc/OS X]: Implement mem_used tracking using custom malloc zone

OS X doesn't allow direct interposing of malloc/calloc/free, instead
it provides the ability to register an alternate malloc zone. We
therefore use this feature to register a 'wrapper' zone, which
forwards all requests to the underlying default malloc, adding in
appropriate calls to any registered new/delete hooks.

Change-Id: I19b7eb36ac10739776c47d1a2b29f6eada9f824e
Reviewed-on: http://review.couchbase.org/42523
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: buildbot <build@couchbase.com>