core: Add AO2_ALLOC_OPT_NO_REF_DEBUG option.

Previous to this patch passing a NULL tag to ao2_alloc or ao2_ref based
functions would result in the reference not being logged under
REF_DEBUG.  This could sometimes cause inaccurate logging if NULL was
accidentally passed to a reference action.  Now reference logging is
only disabled by option passed to the allocation method.

Change-Id: I3c17d867d901d53f9fcd512bef4d52e342637b54