MB-12608: Enable tcmalloc.aggressive_memory_decommit by default

Enable TCMalloc's 'aggressive decommit' mode by default.

Change-Id: Ia18d1e37e784a041d686d0001eda588f8f079dc4
Reviewed-on: http://review.couchbase.org/42901
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>