Revision 1570659

PROTON-489: Use reference counting for object management.

Each child object will keep a reference to its parent: Deliveries
reference Links, which reference their Sessions, which reference the
containing Connection.  This keeps the parent objects present until
all child objects have been released.

The transport work lists also reference count their members (Endpoints
and Deliveries).  This keeps freed objects around long enough to flush
their pending state prior to deleting them.