This commit is related to #27260. Currently there is a weird
relationship between channel contexts and nio channels. The selectors
use the context for read and writing. But the selector operates directly
on the nio channel for registering, closing, and connecting.
This commit works on improving this relationship. The selector operates
directly on the context which wraps the low level java.nio.channels. The
NioChannel class is simply an API that is used to interact with the
channel (sending messages from outside the selector event loop,
scheduling a close, adding listeners, etc). The context is only used
internally by the channel to implement these apis and by the selector to
perform these operations.
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