Make IOCP rate-limiting group support stricter and less surprising.

Make IOCP rate-limiting group support stricter and less surprising.

Previously, we wouldn't decrement read/write buckets because of IOCP
reads and writes until those reads and writes were complete.  That's
not so bad on the per-connection front.  But for group limits, the
old approach makes us launch a huge amount of reads and writes
whenever the group limit becomes positive, and then decrement the
limit to a hugely negative number as they complete.

With this patch, we decrement our read buckets whenever we launch an
IOCP read or write, based on the maximum that tried to read or
write.  Later, when the operations finish, we re-increment the
bucket based on the portion of the request that couldn't finish.