Make bgp_stress_test and bgp_peer_close_test fail less often.

A few underlying issues are fixed.

- Remove incorrect assertion in BgpXmppChannel::ReceiveUpdate.
- Disable graceful restart related tests for now.
- Make NetworkAgentMock::Delete wait for XmppConnections and
TcpSessions to get cleaned up.
- Implement NetworkAgentMock::SessionDown by forcing the XMPP
client state machine into Idle state instead of just deleting
the AgentPeer.
- Shutdown the TcpServer right away instead of doing it from
the bgp::Config task when terminating XmppServer/XmppClient.

The change to control_node.cc is cosmetic i.e. task exclusion
policy is unchanged.

Change-Id: I090f08400d9c327df2dd59774c87fac3087fd52c
Merge "Make bgp_stress_test and bgp_peer_close_test fail less often."