Implement introspect command to show xmpp connections

Add a new XmppSandeshContext structure so that the xmpp code does
not have any dependency on bgp. Note that struct BgpSandeshContext
should really be named something like ControlNodeSandeshContext.

Change-Id: Idb87b767192a20a80be3925ddd05d409a88249ad
Closes-Bug: 1389453