Astobj2: Add ao2_weakproxy_ref_object function.

This function allows code to run ao2_ref against the real
object associated with a weakproxy.  It is useful when
all of the following conditions are true:
* You have a pointer to weakproxy.
* You do not have or need a pointer to the real object.
* You need to ensure the real object exists and is not
  destroyed during a process.

In this case it's wasteful to store a pointer to the real
object just for the sake of releasing it later.

Change-Id: I38a319b83314de75be74207a8771aab269bcca46