Add a couple more tests for deferred xmpp peer close.

This covers scenarios where an xmpp peer has pending subscribe or
unsubscribe requests to the membership manager when xmpp channel
gets closed. We wait for completion notifications for outstanding
requests from the membership manager before triggering peer close
procedures.

Also add more peer membership verifications to existing tests.
Merge pull request #575 from Juniper/nsheth_xmpp_deferq

Add a couple more tests for deferred xmpp peer close.