Fix serious bugs in per-bufferevent rate-limiting code

Fix serious bugs in per-bufferevent rate-limiting code

Our old code was too zealous about deleting the refill events that
would actually make connections able to read or write again after
they had run out of bandwidth.  Under some circumstances, this could
cause a bufferevent to never actually refill one of its
rate-limiting buckets.

Also, the code treated setting a per-connection rate-limit on a
connection that already had a group-limit as if it were changing the
limit on a connection whose allocation had already run out.

This patch fixes both of those problems.