We need these for Tor, and other projects probably need them too. Uses
include:
- Checking whether bandwidth is mostly-used, and only taking some
actions when there's plenty of bandwidth.
- Deducting some non-bufferevent activities from a rate-limit group.
Most of these should be unable to fail, since adding a timeout
generally always works. Still, it's better not to try to be "too
smart for our own good here."
There are some remaining event_add() calls that I didn't add checks
for; I've marked those with "XXXX" comments.
The fairness algorithms are not the best, not every bufferevent type
is supported, and some of the locking tricks here are simply absurd.
Still, this code should be a good first step.
