Make cdr_mysql compile again by not directly setting the run-time CDR object

Make cdr_mysql compile again by not directly setting the run-time CDR object

A stray ast_cdr_setvar was missed in cdr_mysql (silly addons). This has now
been refactored to not set the property, as the property would have been set
on a run-time object that was already dispatched to the backend. The module
simply remembers the value it wanted to set and writes it to MySQL later
in the processing.

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@391964 65c4cc65-6c06-0410-ace0-fbb531ad65f3