Fix handling of group rate limits under 64 bytes of burst

Fix handling of group rate limits under 64 bytes of burst

The "min_share" logic, which was designed to prevent piles of
extremely small writes when running up against a group rate limit,
could lead to confusing behavior if you ever set a min_share less
than your burst rate.  If that happened, then as soon as your group
rate limit was exhausted, you'd stop reading/writing, and never
start again, since the amount readable/writeable would never
actually hit min_share.

We now cap min_share at the rate per tick.

Found by George Kadianakis