Create a better-designed system for handling computation outside the event loop

[nickm]

Right now, we do a couple of things in run_main_loop_once that happen outside the event loop (because we want to re-scan for events event loop before they happen):

• Making events on active_linked_connection_lst active.
• Running connection_ap_attach_pending.

But we can do this much better. With Libevent 2.1, instead of making the loop exit for this, we can should do all of these things in a separate event callback, and call event_base_loopcontinue() at the end of that event's callback so that the event_base will get rescanned before we return. With earlier versions of Libevent, we can do something similar with event_base_loopbreak().

Doing this won't lower the number of wakeups we do, but it should simplify our overall event loop logic, and make other event loop simplifications easier.

[dgoulet]

Setting master ticket parent ID.

[nickm]

I've started work here in a branch called postloop_callbacks based on isolate_libevent_2. It still needs tests and a changes file, and we'll need to rebase it once isolate_libevent_2 is squashed and merged.

[nickm]

Now that isolate_libevent_2 is merged, this branch can get reviewed. Please see postloop_callbacks_2 on github and tor-git.

[nickm]

Github pr is ​https://github.com/torproject/tor/pull/41

[nickm]

per dgoulet's request, I've rebased the branch onto master, and force-pushed it.

[dgoulet]

Let a comment on the PR: ​https://github.com/torproject/tor/pull/41#issuecomment-381180728

This is merge_ready after that imo.

[nickm]

Resolved issue; merging now!