prop224: Use the intro point state cache

This commit makes the client use the intro point state cache. It notes down
when we get a NACK from the intro point and then uses that cache to decide if
it should either close the circuits or re-extend to a new intro point.

This also introduces a very useful function that checks if an intro point is
usable that is query the state cache and checks a series of requirement.

Signed-off-by: David Goulet <dgoulet@torproject.org>