We handle RESOLVED cells incorrectly on half-closed streams.

Recall that a half-closed stream is one where we have sent an END cell, but not yet received an acknowledging END.

The protocol allows us to send END on a stream where we have send RESOLVE, in order to cancel the request. But if we get back a RESOLVED on such a stream, we'll treat it as a protocol violation. That's incorrect!

Found while working on #525 (closed).