[empiredan]

This PR is to resolve the issue #1066.

The root cause of issue #1066 is that the percentile is freed during the destruction of the metric registry, after which sometimes the handler for asynchronous wait operation of timer will be still executed, where the freed percentile is certainly referenced. This will lead to heap-use-after-free error.

The solution for this problem will be extend the lifetime of percentile: during the construction of the percentile, it can increment its ref count (its base class is ref_counter) since it is referenced by timer. This effectively extends the lifetime of percentile: even if both of its metric entity and registry are destructed, it can still survive in the memory for the reason that its ref count is 1 at least.

Once the percentile should be released when it will retire from metric entity or the metric registry (singleton) will be destructed, the close() method for percentile will be invoked. close() will change the state of timer from running to closing, and also call cancel() for the timer. Then, whether the timer is cancelled successfully or not, the timer will finally be stopped: if successful, the callback for cancel will be executed immediately; otherwise, the closing state will tell the timer to stop finally, albeit sometimes after a while.