Add lcb_UN and lcb_SN for fixed width typedefs

Change-Id: I2e0afb633c89752454370e35a55ee4f773469187
Reviewed-on: http://review.couchbase.org/36274
Reviewed-by: Brett Lawson <brett19@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>