2.x.x http2 idle state (version 1)

[adam-fowler]

I've implemented HTTP2 idle state using the IdleStateHandler and custom HTTP2 stream open and close events.
The idea being you shutdown when there are no open streams (it is up to the open streams to handle idle state) and an read idle event has been triggered.
The standard HTTP2 events are not generated by the channel handlers created by configureAsyncHTTPServerPipeline so I have to create custom events and pass them back to the channel as outbound events to say when streams open and close..

An alternative method can be found in #371

[codecov]

Codecov Report

Attention: 36 lines in your changes are missing coverage. Please review.

Comparison is base (e13bb01) 84.32% compared to head (860010a) 83.96%.

Files	Patch %	Lines
...urces/HummingbirdHTTP2/HTTP2UserEventHandler.swift	61.11%	35 Missing ⚠️
Sources/HummingbirdHTTP2/HTTP2Channel.swift	95.45%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##            2.x.x     #370      +/-   ##
==========================================
- Coverage   84.32%   83.96%   -0.37%     
==========================================
  Files          95       96       +1     
  Lines        5211     5312     +101     
==========================================
+ Hits         4394     4460      +66     
- Misses        817      852      +35     

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