SA: CollectedHeap provides broken implementation for used() and capacity()

The CollectedHeap class provides concrete (broken) implementations for used() and capacity(). I propose that we make these functions abstract to force the sub classes to provide a correct implementation.

This was found while running serviceability/sa/TestHeapDumpForLargeArray.java on an experimental implementation of heap dumping in ZGC.

ZGC didn't provide a ZCollectedHeap.used() function and CollectedHeap.used() was used instead at:
        // Check weather we should dump the heap as segments
        useSegmentedHeapDump = vm.getUniverse().heap().used() > HPROF_SEGMENTED_HEAP_DUMP_THRESHOLD;

Because of this we incorrectly did not use segmented heap dumps, and therefore overflowed later in the code.