Implement ConcurrentTimerSet + RefHolder.Hold / Release on top if it about stl.fusion

[alexyakunin]

Possible impl:

• Each level = ConcurrentDictionary<TKey, Moment>[BucketSize] (cycle buffer)
• Extra ConcurrentDictionary<TKey, (Moment, int)> points to a level where a specific timer "lives"; any operation runs on this dict first, and if succeeds, it seamlessly proceeds to the level dict.
• An array of indexes (int[]) points to "zero bucket" in each level - plus we need an extra Moment[] array to track each level's "zero bucket" start Moment.
• A background task runs every MinDelay to move above pointers & fire timers.

[alexyakunin]

I.e. as a result, we get timer implementation that has Max(MinDelay, delay/BucketCount) precision. This is usually fine for things like keep alive delays + it should be fairly fast (~ zero alloc. per timer assuming max. timer count is fixed) & scale well with core count.

[alexyakunin]

Implemented more efficiently (with Radix Heap) in this commit: f6cf75a

[alexyakunin]

And in this commit: 8dfe389#diff-557e384208e4a49b1f0275a4bc138dcd