circuit::send_control_message: mismatch between documentation and behavior

I've looked over the documentation and the behavior for send_control_message while trying to figure out pending bugs on !1228 (merged).

As near as I can see, the documentation plainly says that send_control_message should not return until the handler has uninstalled itself.

But the implementation does returns as soon as the message is sent and the handler is installed.

We should pick one of these as the better behavior, and change the other to match.