sllist: Add insert_sorted()

Change-Id: I0a70a4be1fae8184b0eb56125e89f2c3b0f239c2
Reviewed-on: http://review.couchbase.org/34664
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Brett Lawson <brett19@gmail.com>