Fix: crmd: Consider only fence and shutdown transition events to be "down" events

Before, match_down_event() only ignored CRM_OP_LRM_REFRESH events, and would
sometimes match CRM_OP_CLEAR_FAILCOUNT. Now, it ignores everything except
CRM_OP_FENCE and CRM_OP_SHUTDOWN.

Also, a scary debug message was made into a kinder, gentler trace message.