MB-12608: Add support to en/disable tcmalloc.aggressive_memory_decommit dynamically

Add two new commands to mcctl to get/set TCMalloc's aggressive memory
decommit mode dynamically.

Usage:

    Get current aggressive memory decommit setting:
        mcctl get tcmalloc.aggressive_memory_decommit

    Enable agressive decommit:
        mcctl set tcmalloc.aggressive_memory_decommit 1

    Disable agressive decommit:
        mcctl set tcmalloc.aggressive_memory_decommit 0

Change-Id: I6bb30c23b18fe58b69365421a4a485148662e805
Reviewed-on: http://review.couchbase.org/43055
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: buildbot <build@couchbase.com>