`NIOHTTP2Handler` should handle `ChannelShouldQuiesceEvent`

[fabianfett]

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SwiftNIOExtras offers a QuiescingHelper to gracefully shutdown servers. NIOHTTP2Handler should support shutting down a server connection, when the QuiescingHelper sends out a ChannelShouldQuiesceEvent:

• Once NIOHTTP2Handler receives a ChannelShouldQuiesceEvent a GoAway frame should be send to the client and new incoming streams should be rejected.
• Once all previously open streams have finished the connection should be closed.

In NIOHTTP1 the ChannelShouldQuiesceEvent support is implemented in HTTPServerPipelineHandler

[lmcd]

Hmm maybe this is why I was having issues performing a soft-restart of a HTTP/2 Vapor server. I remember seeing quiesce/shutdown errors.