hs: Update intro circuit timestamp when re-extending

A client can re-extend up to 3 intro points on the same circuit. This happens
when we get NACKed by the intro point for which we choose a new intro and
re-extend the circuit to it.

That process can be arbitrarly long so reset the dirty timestamp of the
circuit everytime we choose to re-extend so we get a bit more time to actually
do our introduction.

This is a client circuit so it is short live once opened thus giving us a bit
more time to complete the introduction is ok.

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