This speeds up the test suite significantly by reducing the overhead of
spawning a new process.

Notify client close

This inserts a callable `waitress.client_disconnected` into the
environment that allows the task to check if the client disconnected
while waiting for the response at strategic points in the execution,
allowing to cancel the operation.

It requires setting the new adjustment `channel_request_lookahead` to a
value larger than 0, which continues to read requests from a channel
even if a request is already being processed on that channel, up to the
given count, since a client disconnect is detected by reading from a
readable socket and receiving an empty result.

This may happen on macOS