Fix a nasty dangling-event bug when using rate-limiting groups

Fix a nasty dangling-event bug when using rate-limiting groups

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.