7129514: time warp warnings after 7117303 Summary: Replace calls to os::javaTimeMillis() that are used to update the milliseconds since the last GC to an equivalent that uses a monotonically non-decreasing time source. Reviewed-by: ysr, jmasa

7129892: G1: explicit marking cycle initiation might fail to initiate a marking cycle Summary: If we try to schedule an initial-mark GC in order to explicit start a conc mark cycle and it gets pre-empted by antoher GC, we should retry the attempt as long as it's appropriate for the GC cause. Reviewed-by: brutisso, johnc