Someday, when networks are far faster and people frequently want a
burst value greater than 2GB per tick, this will seem very forsightful
indeed.
For now, it breaks ABI, but not source. Fixes bug 3092096.
When we freed a bufferevent that was in a rate-limiting group and
blocked on IO, the process of freeing it caused it to get removed
from the group. But removing the bufferevent from the group made
its limits get removed, which could make it get un-suspended and in
turn cause its events to get re-added. Since we would then
immediately _free_ the events, this would result in dangling
pointers.
Fixes bug 3041007.
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.
