xds: avoid log spam during server mode switches (better A36 compliance)

[easwars]

Summary of changes:

• Suppress redundant mode changes in the listenerWrapper
• Throw a log entry from the xDS server if and only if a mode change callback is not registered
• Throw a single log entry from the mode change callback registered by the xDS interop server

Test changes:

• Register a mode change callback from integration tests to get around our testLogger failing the test
• Replace some calls to t.Errorf() with t.Fatalf()

Fixes #4939

RELEASE NOTES: none

[easwars]

@dfawley : I made the change we discussed offline to register a callback to log the mode changes if the user did not register any. And therefore, we can directly invoke the callback without a nil check. PTAL.

[easwars]

@dfawley : Ping ...

[dfawley]

Make this a standalone function?

[easwars]

Done.

[dfawley]

Should we have a defaultServerOptions that has this set in it, such that if applying the options doesn't override it, we get the defaults? (e.g. how we handle DialOptions in the grpc package)

[easwars]

Done.

[ejona86]

"Suppress redundant mode changes in the listenerWrapper" doesn't sound right. If things are continuing to fail, we need to continue to notify. This is because 1) the error may have changed and 2) it provides a timestamp of the last backoff attempt. The only time to "suppress redundant mode changes" is in the successful case where we are listening and the new mode is still listening.