Check if a stream could send a cell but the circuit hasn't asked it to
Our #snackbug friend may have encountered a situation where somehow a circuit decides to never check again whether a stream had something to say, so when the circuit could send more, it doesn't, until the next time it gets told to check if any streams are ready to send.
For example, the exit relay waits with its 'connected' relay cell until a new stream is created, and then it suddenly notices that it had one it wanted to send.
Seems like we should sort out exactly what the invariant is supposed to be ("if the package window is at least this, and the cell queue is less than this water mark, then it should never be the case that a stream on that circuit has something ready to send but hasn't sent it"), and check it in some smart places and log if it's violated.