cdr.c: Eliminated many calls to ao2_global_obj_ref().

cdr.c: Eliminated many calls to ao2_global_obj_ref().

The CDR performance gets worse the further it gets behind in processing
stasis messages.  One of the reasons is we were getting the global config
to determine if we needed to log a debugging message.

* Many calls to ao2_global_obj_ref() were just so we could determine if
debug mode is enabled.  Made a global flag to check instead.

* Eliminated many RAII_VAR() usages associated with the remaining
ao2_global_obj_ref() calls.

* Added missing NULL checks for the returned ao2_global_obj_ref() value.

ASTERISK-27335

Change-Id: Iceaad93172862f610cad0188956634187bfcc7cd