Astobj2: Allow reference debugging to be enabled/disabled by config.

* The REF_DEBUG compiler flag no longer has any effect on code that uses
  Astobj2.  It is used to determine if reference debugging is enabled by
  default.  Reference debugging can be enabled or disabled in asterisk.conf.
* Caller information is provided in logger errors for ao2 bad magic numbers.
* Optimizes AO2 by merging internal functions with the public counterpart.
  This was possible now that we no longer require a dual ABI.

ASTERISK-24974 #close
Reported by: Corey Farrell

Change-Id: Icf3552721fe999365ba8a8cf00a965aa6b897cc1