client/comms: expire requests on connection shutdown

[chappjc]

This avoids hangs on shutdown with comms requests that would be waiting to timeout.

Since the connection dropping does not abort requests (and should not since there is a reconnect loop), we must call the expire func when the wsConn is explicitly shutdown, otherwise users of Request and RequestWithTimeout may hang until the request timeout is reached, depending on the use pattern.

For example, with this pattern used throughout the client, it would hang at <-errChan until timeout even if the conn were explicitly shutdown after sending the request:

errChan := make(chan error, 1)
err = conn.RequestWithTimeout(reqMsg, func(msg *msgjson.Message) {
	errChan <- msg.UnmarshalResult(response)
}, timeout, func() {
	errChan <- fmt.Errorf("timed out waiting for %q response", route)
})
// Check the request error.
if err != nil {
	return err
}
// Check the response error.
return <-errChan