Actually decrement the stream-level SENDME window

!126 (merged) 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 (closed)