Start mixed GC on the next GC after marking cycle completed

The G1 collection cycle does an additional young-only GC between the cleanup pause and the first mixed gc. In some cases, in relatively tight heaps, this makes the mixed gcs start too late, causing to-space overflows and in the end results in undesired full gcs.

There does not seem to be a good reason to do the young-only gc after GC cleanup at all.

Fix this to switch to mixed gcs right after the marking cycle completes.

An "intermediate" fix could be to decrease the IHOP slightly.

A program that shows this problem is e.g. CRM Fuse.