Our old "do we need to launch a circuit for stream S" logic was,
more or less, that if we had a pending circuit that could handle S,
we didn't need to launch a new one.

But now that we have streams isolated from one another, we need
something stronger here: It's possible that some pending C can
handle either S1 or S2, but not both.

This patch reuses the existing isolation logic for a simple
solution: when we decide during circuit launching that some pending
C would satisfy stream S1, we "hypothetically" mark C as though S1
had been connected to it.  Now if S2 is incompatible with S1, it
won't be something that can attach to C, and so we'll launch a new
stream.

When the circuit becomes OPEN for the first time (with no streams
attached to it), we reset the circuit's isolation status.  I'm not
too sure about this part: I wanted some way to be sure that, if all
streams that would have used a circuit die before the circuit is
done, the circuit can still get used.  But I worry that this
approach could also lead to us launching too many circuits.  Careful
thought needed here.