xds: Cleanup CDS balancer code and tests.

[easwars]

• Code cleanup:

  • Move comments from function top to near the actual code
  • Setup a no-opcancelWatch so that we don't need nil checks before invoking it

• Tests cleanup:

  • Pass ctx with deadline to functions which wait for certain events. This way callers control the deadline.
  • Add more comments in every test to make them more readable on their own.
  • Add more tests for Close
  • Use a shorter deadline for events which are expected to not happen

• With this change:

  • Test times reduce by 10x
  • Code coverage increases by 5%

[easwars]

I started working on the CDS balancer changes to support balancer.ClientConn interface, but found that the tests weren't very readable. So, I thought I will first fix this and then make my changes.

[easwars]

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