Bump default value of G1RefProcDrainInterval

The G1RefProcDrainInterval flag determines the frequency at which new marks are actually processed while keeping alive references during reference processing.

The current value is 10, i.e. every ten references we try to drain the mark stack. However, due to how expensive setup and teardown of G1 marking is, this seriously inhibits performance.

Tests showed that with a significantly higher value, 1000, reference processing time can halve.