Handle call cancellation gracefully

[sschepens]

Fixes #79

Immediately cancel watches when responseObserver is cancelled. Catch cancellation exceptions when responding watches. Correctly handle cancellations on error.

[sschepens]

@snowp @joeyb i'm inclined to remove WatchCancelledException from Watch#respond it seems as though we sould handle cancellations gracefully as they occur in normal situations, and handling them through exceptions doesn't seem right, too much overhead.

What do you think?

[sschepens]

I'm also not entirely sure how to test this changes, I don't know how to trigger a cancellation through tests

[snowp]

To test I think you want to call onError(Status.CANCELED) on the client stream observer. From what I can tell this will send an RST_STREAM to the server which it will treat as a canceled stream

[codecov-io]

Codecov Report

Merging #80 into master will decrease coverage by 0.42%. The diff coverage is 94.87%.

@@             Coverage Diff              @@
##             master      #80      +/-   ##
============================================
- Coverage     94.82%   94.39%   -0.43%     
- Complexity      130      131       +1     
============================================
  Files            13       13              
  Lines           502      517      +15     
  Branches         44       46       +2     
============================================
+ Hits            476      488      +12     
- Misses           18       21       +3     
  Partials          8        8

Impacted Files	Coverage Δ	Complexity Δ
...nvoyproxy/controlplane/server/DiscoveryServer.java	96.29% <94.87%> (-2.63%)	13 <0> (+1)

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[sschepens]

@snowp added a test for this, please review