Put all the possible assert() we can do on a client introduction circuit in
one helper function to make sure it is valid and usable.
It is disabled for now so gcc doesn't complain that we have a unused function.
Signed-off-by: David Goulet <dgoulet@torproject.org>
This commit paves the way for the e2e circuit unittests.
Add a stub for the prop224 equivalent of rend_client_note_connection_attempt_ended().
That function was needed for tests, since the legacy function would get
called when we attach streams and our client-side tests would crash with
assert failures on rend_data.
This also introduces hs_client.[ch] to the codebase.
