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: I7dbbdd084188a1781cace3166e3a8125068dfd4d
Closes-Bug: 1389453