Overhaul of `online_reps` class

[pwojcikdev]

This is an overhaul of online_reps class:

• Run periodic online weight sampling on a dedicated thread, which makes it more robust and easier to debug.
• Eliminate use of store.online_weight.count () and erasing entries based on their timestamps instead (2 weeks for live, 1 day for beta).
• Add stats and logging to improve observability.
• Add asserts to ensure data consistency.
• Split and rewrite functions, so they are (subjectively) cleaner.
• Ensure that nano::online_reps::trended () can never return value lower than the limit configured via online_weight_minimum.