Make `serve()` an async context manager

[jlaine]

In order to get a clean shutdown of the asyncio-based QUIC server, we need to:

• Make the QuicServer.close() method async.
• Make the serve() method an async context manager.

See #513

[codecov[bot]]

Codecov Report

:white_check_mark: All modified and coverable lines are covered by tests. :white_check_mark: Project coverage is 100.00%. Comparing base (352bcf8) to head (e370663). :warning: Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #515   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           25        25           
  Lines         5102      5108    +6     
=========================================
+ Hits          5102      5108    +6     

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[jlaine]

@rthalley This is a breaking change, so it looks we would need to make the next release 2.0.

[rthalley]

I think clean closing is worth the breaking change.

[jlaine]

@rthalley I think I'm going to merge this, any objection?

[HarshArora-1205]

Status?