Make xmmp tests more robust.

Following changes are implemented:

- Fix race condition in updating XmppServer connection count
- Add xmpp peer configured/negotiated hold times to introspect
- Lay groundwork for xmpp hold time negotiation
- Use consistent APIs to clear one or all compute node connections
- Remove unused method XmppChannelCleanup from XmppChannel
- Simplify connection management code in XmppServer and XmppClient
- Move xmpp_session_test from flaky to normal

Also make a number of cosmetic changes to modified files.

Change-Id: If8720c4ed9c0755ec30e1b096ae5b8a9536c3381
Merge "Make xmmp tests more robust."