Improve graceful shutdown

[WaffleLapkin]

Turns out we were wasting a lot of time for no reason while doing graceful shutdown, this PR fixes it. This almost fixes #711 — the shutdown is fast enough that it isn't really frustrating. Sometimes telegram still takes a bit of time to respond to the last get_updates request¹, so I think we should still implement the "multiple ^C for forceful shutdown" (additionally it would help for cases when handlers are stuck).

Here is a comparison of new version (this PR/branch) and old one (current master)²³:

Peek.2023-09-25.22-05.mp4

Footnotes

1. I think it happens when last get_updates call and the previous one happen with little delay between them ↩

2. Note that in second runs of both versions a wait for like 10s or so and the shutdown is much faster for those cases, that's the telegram quirk, I think ↩

3. I'm so dead by the fact that the new version is so much faster⁴, like... we could have had this all along 💀 we received so many complaints about the slow shutdown, and it turns out it was almost entirely our fault 💀 ↩

4. the first run is 18s -> 3s (6x times faster), the second one is 10s (11s waiting) -> less than a second (8s waiting) (that's so much faster aaaaa) ↩

[hirrolot]

Nice, will have a look at it ASAP.

[WaffleLapkin]

cc @Studiedlist this is semi-related to your work in #894