Refactor task_timer to allow mocking the timer

[StefanoPetrilli]

The class task_timer uses std::chrono::steady_clock. Currently, in the tests the real time has to elapse which makes running the test suite take much longer than it could because of the tests regarding this component.

I propose refactoring task_timer to something like:

template <typename ClockType = std::chrono::steady_clock>
class task_timer {
...
}

This would allow instantiating the task_timer using a mock timer and would allow replacing this_thread::sleep_for(chrono::seconds(4)); with something along the line of timer.advance(4); effectively making all the tests involving timers instant to execute.

[gittiver]

What about giving the interval time to the constructor (with a default of 1s)?