Allow for easy selection of memory backed to use

Change-Id: I5b2b88a0c68b233252b0673a3ed89bbbdb91707f
Reviewed-on: http://review.couchbase.org/37959
Reviewed-by: Dave Rigby <daver@couchbase.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>