chrono: handystats::chrono interface has been reworked.

- clock concept, clock::time_point and clock::duration try to follow std::chrono interface.
- clock::time_point to std::chrono::system_clock::time_point conversion added.
- handystats::chrono::duration_cast resplaces std::chrono::duration_cast
metrics: parameters for timer operations have been swapped (timestamp, instance -> instance, timestamp).

Preferred way to use timer is without specifying timestamp (default value is current time),
while specifying timer's instance is more commonly used.
tests: helper function to wait for handystats::message_queue to become empty has been added.