Feature Request: concurrent update handling in dispatching2

[Hirrolot]

Apparently, it turns out that dispatching2 handles all updates sequentially. This is undesired for obvious reasons. I suggest making it concurrent to allow proper scaling with a growing number of bot users.

[Hirrolot]

Also, don't forget that updates from the same chats should be handled sequentially, one by one. We can use something like flurry to associate chat IDs to Tokio channels handling updates.

[WaffleLapkin]

Implemented by #585 and #589