We don't need parent or child ids unless debugging. IDLE and EXPIRED
conn logs do not need to report ids either. Ids are useful only in
detailed debug logs.
This prevents possible multithreading issues between thrmgr and conn
handling threads. So we can remove and clean up the code and comments
related with such possible issues now. For example, we can add the conn
to its thread list earlier, and we can handle errors immediately, thanks
to this early switch to conn handling threads. This also helps achieve
cleaner code.