Handle case where RoutingInstance is deleted when processing subscribe.

Previously we would go ahead and process the subscribe even if routing
instance was marked deleted.  Some of the tables in the instance may
already have been deleted.  This would obviously cause problems later
since the agent wouldn't really have been subscribed per the membership
manager.

Fix is to add instance to the vrf membership request map and process it
as a deferred subscription request when the instance gets created later.

Also take care of the case where we get an unsubscribe when the instance
is marked deleted.  If the instance is not in the vrf membership request
map, we process the unsubscribe as normal.  Otherwise we just remove it
from the vrf membership request map and flush any entries on the DeferQ.

Add tests to recreate the issue and verify the fix.
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.
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.
Test subscribe/unsubscribe to two different incarnations of instance.

The first subscribe/unsubscribe are processed after the table has been
marked as deleted but has not yet been destroyed. The second subscribe
is received before the table and instance have been destroyed. Later,
the old table and instance are destroyed and new incarnations created.
The subscribe needs to happen on the new incarnation.
Merge pull request #594 from Juniper/nsheth_r105_xmpp_deferq

Cherry pick fixes for xmpp subscribe/unsubscribe with deleted instances.