The main.c code is responsible for initialization and shutdown;
the mainloop.c code is responsible for running the main loop of Tor.
Splitting the "generic event loop" part of mainloop.c from the
event-loop-specific part is not done as part of this patch.
Since we're going to be disabling the second-elapsed callback, we're
going to sometimes have long periods when no events file, and so the
current second is not updated. Handle that by having a better means
to detect "clock jumps" as opposed to "being idle for a while".
Tolerate far more of the latter.
Part of #26009.
