Non-blocking IO

[compscidr]

Previously we were reading and writing from the socket on two different threads at the same time which led to deadlock quite often.

This switches the IO to use a selector following this guide: https://rox-xmlrpc.sourceforge.net/niotut/ where the selector is only ever modified by a single thread.

By default, we are always in read mode on the remote socket, and then when data is ready, we switch to read mode and write.

I also updated the jdk to java 21.

[codecov]

Codecov Report

Attention: Patch coverage is 71.42857% with 58 lines in your changes missing coverage. Please review.

Project coverage is 63.32%. Comparing base (de306d0) to head (5d3852c).
Report is 9 commits behind head on main.

Files with missing lines	Patch %	Lines
...c/main/kotlin/com/jasonernst/kanonproxy/Session.kt	76.15%	15 Missing and 16 partials ⚠️
...m/jasonernst/kanonproxy/tcp/AnonymousTcpSession.kt	58.97%	14 Missing and 2 partials ⚠️
...kotlin/com/jasonernst/kanonproxy/udp/UdpSession.kt	63.15%	5 Missing and 2 partials ⚠️
.../jasonernst/kanonproxy/BidirectionalByteChannel.kt	42.85%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main      #43      +/-   ##
============================================
+ Coverage     61.46%   63.32%   +1.85%     
- Complexity      196      221      +25     
============================================
  Files            14       15       +1     
  Lines          1783     1903     +120     
  Branches        263      283      +20     
============================================
+ Hits           1096     1205     +109     
+ Misses          529      526       -3     
- Partials        158      172      +14     

Flag	Coverage Δ
libunittests	63.32% <71.42%> (+1.85%)	⬆️

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