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Background: in 2019 in 9c260fde (time: New package that mirrors Go's
time) and b073f6df (time: Move/Port timers to C++/Pyx nogil) I've added
basic timers - with proper API but with very dumb implementation that
was spawning one thread per each timer. There were just a few timers in
the users and this was working, surprisingly, relatively ok...

... until 2023 where I was working on XLTE that needs to organize 100Hz
polling of Amarisoft eNodeB service to retrieve information about flows
on Data Radio Bearers:

    kirr/xlte@2a016d48
    https://lab.nexedi.com/kirr/xlte/-/blob/8e606c64/amari/drb.py

There each request comes with its own deadline - to catch "no reply",
and the deadlines are implemented via timers. So there are 100 threads
created every second which adds visible overhead, consumes a lot of
virtual address space and RSS for threads stacks, and should be all unnecessary.

We was tolerating even that for some time, but recently Joanne ap...