6994297: G1: do first-level slow-path allocations with a CAS Summary: First attempt to allocate out the current alloc region using a CAS instead of taking the Heap_lock (first level of G1's slow allocation path). Only if that fails and it's necessary to replace the current alloc region take the Heap_lock (that's the second level of G1's slow allocation path). Reviewed-by: johnc, brutisso, ysr