!126 overhauled the `tor-proto` circuit reactor, but left out one
very important thing: actually decrementing the SENDME window for
streams (not circuits) when we send cells along them.

Since the circuit-level SENDME window would often prevent us from
running into a problem, this wasn't caught until my benchmarking efforts
noticed it (in the form of Tor nodes aborting the circuit for a protocol
violation).

fixes #260