First, this commit moves the code used to prune the service list when
reloading Tor (HUP signal for instance) to a function from
rend_config_services().
Second, fix bug #21054, improve the code by using the newly added
circuit_get_next_service_intro_circ() function instead of poking at the global
list directly and add _many_ more comments.
Fixes#21054.
Signed-off-by: David Goulet <dgoulet@torproject.org>
