6797870: Add -XX:+{HeapDump,PrintClassHistogram}{Before,After}FullGC Summary: Call newly created CollectedHeap::dump_{pre,post}_full_gc before and after every stop-world full collection cycle on GenCollectedHeap and ParallelScavengeHeap. (Support for G1CollectedHeap forthcoming under CR 6810861.) Small modifications to existing heap dumping and class histogram implementation, especially to allow multiple on-the-fly histos/dumps by the VM thread during a single safepoint. Reviewed-by: jmasa, alanb, mchung

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