RFR: Two small changes to generational partial collections...

[Christine Flood]

Heuristics for partial gc included:
if (used < prev_used) {
      // Major collection must have happened, "used" data is unreliable,
wait for update.
      return false;
    }

Except we weren't updating prev_used after full concurrent collections so
we would have long dry spells with no partial collections.

We were only looking at the desired number of regions when building partial
collection sets, but because we were rejecting some (in some cases many) we
were ending up with collection sets that were too small.  This allows us to
keep going until we either run out of regions or reach our desired
collection set size.

http://cr.openjdk.java.net/~chf/SmallFixes/webrev/


Christine