astobj2: Add support for weakproxy objects.

This implements "weak" references.  The weakproxy object is a real ao2 with
normal reference counting of its own.  When a weakproxy is pointed to a normal
object they hold references to each other.  The normal object is automatically
freed when a single reference remains (the weakproxy).  The weakproxy also
supports subscriptions that will notify callbacks when it does not point
to any real object.

ASTERISK-24936 #close
Reported by: Corey Farrell

Change-Id: Ib9f73c02262488d314d9d9d62f58165b9ec43c67