prop224: Circuit has opened and ESTABLISH_INTRO cell

prop224: Circuit has opened and ESTABLISH_INTRO cell

Add the entry point from the circuit subsystem of "circuit has opened" which
is for all type of hidden service circuits. For the introduction point, this
commit actually adds the support for handling those circuits when opened and
sending ESTABLISH_INTRO on a circuit.

Rendevzou point circuit aren't supported yet at this commit.

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