Added: cb_mutex_try_enter

Change-Id: Ic87271428da4b14c40b41e8c5b1b09839c0ac0bd
Reviewed-on: http://review.couchbase.org/32420
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>