`NIOHTTP2Handler` should handle `ChannelShouldQuiesceEvent`

[fabianfett]

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.

[glbrntt]

• 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.

This needs to be handled carefully, the server should attempt to allow for 'in-flight' streams to be opened before it begins to reject streams. It can do this by sending a GOAWAY frame with lastStreamID set to .max followed immediately by a PING. On receiving the PING ACK – and thereby knowing the client received the previous GOAWAY frame – the server can send another GOAWAY frame with a lower lastStreamID, and then begin rejecting streams.

The client may for some reason not send a PING ACK which could leave the server waiting indefinitely. As the library doesn't (currently) have a built-in mechanism to retrieve RTT the server should also set a timer as a fallback mechanism.

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In gRPC Swift this is managed by a separate channel handler with a number of different behaviours, many of which are applicable to http servers in general. These include:

• Closing idle connections (where 'idle' means there are no open streams) after a period of inactivity
• Using pings to detect broken connections (typically done when connections have open but infrequently used streams)
• Limiting the maximum age of a connection
• Limiting how long a graceful shutdown can take

As these are generally useful it may make sense to fold these behaviours (along with the graceful shutdown) either into the NIOHTTP2Handler or into a separate channel handler within swift-nio-http2.