This requires the latest Git version of Libevent as of 24 March 2010.
In the future, we'll just say it requires Libevent 2.0.5-alpha or
later.

Since Libevent doesn't yet support hierarchical rate limit groups,
there isn't yet support for tracking relayed-bytes separately when
using the bufferevent system.  If a future version does add support
for hierarchical buckets, we can add that back in.