Investigate default alpha for G1 adaptive IHOP allocation rate

Currently G1 uses an alpha of 0.95 for its adaptive IHOP allocation rate calculation, i.e. the latest value is weighed at 95%.

Behavior seems to be the same with the default value; in order to reduce the amount of magic numbers I propose to use the default 0.7 instead for now.

There is no good explanation of why, and the original change does not clarify either. Investigate whether a better value can be found.