xds/server: create the xDS client when the xDS enabled gRPC server is created

[easwars]

Currently we create the xDS client when Serve() is called for the first time. This leads to unnecessary code complexity since we have to synchronize access to the xDS client and its close function inside the server. The following code paths touch the xDS client:

• In Serve to see if one has been created, and create a new one if none exists
• In Stop and GracefulStop to closed the client
• In the ListenerWrapper to register LDS and RDS watches using it

This PR changes the code such that the xDS client is created when the xDS enabled gRPC server is created. Certain bootstrap configuration validation is also moved to here. Therefore we can fail the server creation when appropriate.

Note that with this change, since the mutex has been eliminated, Serve and Stop can race in the sense that Serve might not return grpc.ErrServerStopped in all cases, but this does not affect any other functionality and does not lead to leaking of the xDS client.

This PR also cleans up the tests to use a real management server and gets rid of using a fake xDS client.

#resource-agnostic-xdsclient-api

RELEASE NOTES: none

[easwars]

@dfawley : The diff in xds/server_test.go is nasty. It might be easier to review that file as though it was a new one instead of using the diff. Thanks.

[easwars]

Fixed test failures. Good for review now.

[dfawley]

Thanks! This looks a lot better and even the tests are a bit simpler, too.