G1: Use mixed GC instead of Full GC to clear out space for failing humongous object allocations

When we fail to allocate humongous objects we sometimes fall back to do a Full GC. Maybe we could choose a number of old regions to evict and do a mixed GC instead. This would probably be much cheaper than doing a full GC.

Example times from Kitchensink logs:
6453.688: [GC pause (G1 Evacuation Pause) (mixed), 0.0301229 secs]
...
6596.389: [GC pause (G1 Humongous Allocation) (young), 0.0322782 secs]
6596.423: [Full GC (Allocation Failure) 1543M->735M(4089M), 4.1359466 secs]