In a number of places, we decrement timestamp_dirty by
MaxCircuitDirtiness in order to mark a stream as "unusable for any
new connections.
This pattern sucks for a few reasons:
* It is nonobvious.
* It is error-prone: decrementing 0 can be a bad choice indeed.
* It really wants to have a function.
It can also introduce bugs if the system time jumps backwards, or if
MaxCircuitDirtiness is increased.
So in this patch, I add an unusable_for_new_conns flag to
origin_circuit_t, make it get checked everywhere it should (I looked
for things that tested timestamp_dirty), and add a new function to
frob it.
For now, the new function does still frob timestamp_dirty (after
checking for underflow and whatnot), in case I missed any cases that
should be checking unusable_for_new_conns.
Fixes bug 6174. We first used this pattern in 516ef41ac1,
which I think was in 0.0.2pre26 (but it could have been 0.0.2pre27).
