Use the UUID API to generate and validate UUIDs for res_calendar_exchange.

Currently the res_calendar_exchange module uses its own method of generating
UUIDs using ast_random().  Now that we have a UUID API we should use that
instead.


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@377974 65c4cc65-6c06-0410-ace0-fbb531ad65f3