Make the default and error handlers Send + Sync

[Hirrolot]

Reduces the pain in the ass when trying to use the dispatcher in tokio::spawn or similar.
Discussion: https://t.me/teloxide/6529.

[WaffleLapkin]

That's technically a breaking change, but it's an overlook and a painful one, so maybe we want to release 0.6.2 with this fix and then yank the 0.6.1?

[Hirrolot]

But v0.6.2 would not indicate a breaking change IIRC. v0.7.0 would, wouldn't it?

[WaffleLapkin]

Yes, you are right. Well that's the trick, if we pretend that 0.6.{0,1} never happened then 0.5->0.6 can break :D

I'm not sure that's a good approach, but at the same time not being able to spawn dispatcher is a noticeable problem which we better fix asap.

[Hirrolot]

Technically speaking, if we release v0.6.2 with this fix, someone's working code might accidentally break (some bots have been already rewritten using dispatching2). Even if we yank v0.6.1.

Maybe it's better to release v0.7.0 with this fix?

[WaffleLapkin]

Sure, let's release it as 0.7

[WaffleLapkin]

Looks good to me. Can you add a test that would show that new Dispatcher is Send+Sync/can be spawned?