add support for read deadlines on the proxied connection

[marten-seemann]

Now this was unpleasant code to write, and I'm not 100% sure it's correct.

We need to implement the deadline functions since quic-go calls them from the quic.Transport.

Write deadlines are tracked in #22.

[codecov]

Codecov Report

Attention: Patch coverage is 90.62500% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 69.68%. Comparing base (ed09a8a) to head (3d20d8b).

Files	Patch %	Lines
conn.go	90.62%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #21      +/-   ##
==========================================
+ Coverage   66.22%   69.68%   +3.46%     
==========================================
  Files           3        3              
  Lines         225      254      +29     
==========================================
+ Hits          149      177      +28     
  Misses         52       52              
- Partials       24       25       +1     

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