The CDR performance gets worse the further it gets behind in processing
stasis messages.  One of the reasons is we were repeatedly setting string
fields to potentially the same string in base_process_party_a().  Setting
a string field involves allocating room for the new string out of a memory
pool which may have to allocate even more memory.

* Check to see if the string field is already set to the desired string.

ASTERISK-27335

Change-Id: I3ccb7e23f1488417e08cafe477755033eed65a7c