prop224: Link rendezvous circuit to edge connection

This commit refactors the handle_hs_exit_conn() function introduced at a prior
commit that connects the rendezvous circuit to the edge connection used to
connect to the service virtual port requested in a BEGIN cell.

The refactor adds the support for prop224 adding the
hs_service_set_conn_addr_port() function that has the same purpose has
rend_service_set_connection_addr_port() from the legacy code.

The rend_service_set_connection_addr_port() has also been a bit refactored so
the common code can be shared between the two HS subsystems (legacy and
prop224).

In terms of functionallity, nothing has changed, we still close the circuits
in case of failure for the same reasons as the legacy system currently does.

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