Fix bug that may cause premature delete of XmppClientConection.

XmppClientConnection::DeleteActor's MayDelete method should not
return true if the XmppClient has no registered event handlers.
It should only look at XmppClientConnection's receiver count.

The above change uncovered a bug in NetworkAgentMock wherein the
AgentPeer constructor registered for xmps::CONFIG on the channel
but never unregistered.

Add methods to XmppServerConnection and XmppClientConnection to
return pointers to XmppServer/XmppClient as appropriate to avoid
static_cast all over the place.

Change-Id: I6140fd342dd5a4a0ed84c4a140cc3b35a61fed1e