CBQE-2400:n1ql string fns

Change-Id: I50bd50d18994f2a1bfdca428a56a24d6216cc332
Reviewed-on: http://review.couchbase.org/41253
Reviewed-by: Ketaki Gangal <ketakigangal@gmail.com>
Tested-by: buildbot <build@couchbase.com>