G1: Only commit actually used space for humongous objects

One problem with humongous objects is internal fragmentation, i.e. the waste of space at the end of the object.

One option is allowing allocation into this area, another simply not committing the whole tail region, and not accounting that uncommitted space in the region budget.

Together with JDK-8229373 this could significantly improve internal fragmentation and relieve memory pressure with (many) humongous objects.

This is somewhat similar to ZGC large object handling within the regionalized heap.