Add "GC" for dispatcher workers

[WaffleLapkin]

We were leaking workers, this PR tries to fix this.

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Solution that we ended up using:

• Dispatcher tracks the current & maximum number of active workers
• If there are more workers than the maximum number of active workers, Dispatcher removes all inactive workers

[WaffleLapkin]

@Hirrolot I've addressed your concern about "gc_worker_count_trigger" parameter. Now teloxide automatically sets this parameter to the maximum number of active workers at the same time.

Pros: you don't need to choose this parameter, it's automatically set to a reasonable value.
Cons: there is no way to decrease this parameter, if normally bot has 1 user, but at peek it has 1000 users, then we are still wasting memory.

IMO this is good-enough as it is and we can improve this algorithm later.