This commit is related to #27260. Right now we have separate read and
write contexts for implementing specific protocol logic. However, some
protocols require a closer relationship between read and write
operations than is allowed by our current model. An example is HTTP
which might require a write if some problem with request parsing was
encountered.
Additionally, some protocols require close messages to be sent when a
channel is shutdown. This is also problematic in our current model,
where we assume that channels should simply be queued for close and
forgotten.
This commit transitions to a single ChannelContext which implements
all read, write, and close logic for protocols. It is the job of the
context to tell the selector when to close the channel. A channel can
still be manually queued for close with a selector. This is how server
channels are closed for now. And this route allows timeout mechanisms on
normal channel closes to be implemented.
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