Migrate Memory::Watchdog to BackgroundTask class

What does this MR do and why?

In !92561 (merged) we added a new helper class BackgroundTask that aims to supersede Daemon.

In this MR I started moving one of the more recent Daemon implementations (Gitlab::Memory::Watchdog) to BackgroundTask.

This makes Watchdog a "normal" Ruby object with all threading-related concerns removed that were previously inherited from Daemon. Instead of implementing template methods inherited from a superclass, we now provide a duck-typed interface via two methods call and start that the BackgroundTask wrapper will dispatch to.

The logic has not changed at all. But tests changed quite a bit: we don't need to sleep anymore because we don't have to wait for threads to run or stop. The main complication left was that call enters a loop, so I had to get "clever" a little in the way I stub out the loop sleep to terminate the task after a given number of iterations. Open for suggestions how that could be improved further to make this more testable.

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Related to #367861 (closed)