Bug ID: JDK-7053537 G1: allocate all HeapRegions in a single block

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This might be a further improvement to what was done in 7045330 (G1: Simplify/fix the HeapRegionSeq class).

Abbreviations:

G1CH : G1CollectedHeap
HR : HeapRegion
HRS : HeapRegionSeq

Before 7045330, we had three levels of indirection from G1CH to a HR instance:

G1CH -> HRS -> HR*[] -> HR

By embedding HRS in G1CH we avoided one of those indirections:

G1CH -> HR*[] -> HR

We might be able to remove one more by allocating the HeapRegion instances in a single HeapRegion[] array instead of the HeapRegion*[] array we have now:

G1CH-> HR[]

Pros:

- One less indirection when mapping address -> region which might be beneficial performance-wise on some platforms.
- Simplified management of the HeapRegion instances (we'll allocate them once and that's it).

Cons:

- We have to pre-allocate the max number of HeapRegions up-front, even if we never grow the heap to the maximum size, which will have some footprint impact.