Handle case where BgpTable is deleted when processing subscribe.

Fix bug in membership manager that caused clients to wait forever
after requesting a register on a deleted table.  We now consider
the registration to be complete right away if the table's deleted
and invoke the client's completion routine.

Add tests to recreate the issue and verify the fix.
Merge pull request #567 from Juniper/nsheth_xmpp_deferq

Handle case where BgpTable is deleted when processing subscribe.