Ensure xdsclient does not crash upon receipt of messages from the management server

[easwars]

We need to do two things here:

• We need to add a recover around the place where we read messages from the ADS stream and process them. This will ensure that even if we get a malformed message or a message with fields that we don't support or contains some invalid values, we don't end up crashing.
• We need to add fuzz tests