#260 Race condition with MCAST creation

[Fabian Tillier]

If a QP is destroyed while a MCAST join operation is
oustanding, the SA operation could attempt to access
the mcast object after it has been cleaned up by the QP.

The MCAST object should be a true al_obj_t type object
to support references and to support binding to the QP
as regular objects do. This greatly simplifies destruction
and error handling.