When switching from a BytesIO to a file backed buffer, we were not
closing the old buffer after we had copied its contents. With this
change when we switch from a small buffer to a larger buffer or
vice-versa we will now attempt to close the old buffer.

The help message contained a misaligned option. This fixes it.

Signed-off-by: Felipe Franciosi <felipe@nutanix.com>

CHANGES.txt entry included.

Instead we just rely on the server_name passed in to the adjustment.

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.