CounterAggregate not counting when cmp_xchg

- For some weird reason, the cmp_xchg is always returning 0 after the
  call. Changed the code to use an std::atomic<double>. Since the
  atomic<double> does not contains fetch_add operation, I am using
  compare_exchange_weak for the atomic operation.

- Removed the cmp_xchg as well from the GaugeAggregator. Now the
  stats_aggregator.h is using only std c++11 atomic operations

This problem might occur, because we are compiling rtbkit with gcc-4.8