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