Errors when cancelling proposal at the very last moment

[nisnevich]

When the proposal is cancelled at the very last seconds (when grant approval coroutine has already started), various errors occur. This should be fixed by checking a lock.

[nisnevich]

Another case:

1. Last voter is clicking to cancel
2. Coroutine starts but doesn't update the message immediately
3. The voter clicks a couple more times

Result: the proposal is cancelled, however exceptions are thrown

Solution: a lock should be associated with proposal id, and checked in on_raw_reaction_add one of the first things

[nisnevich]

Another case:

1. DB recovery in progress
2. User removes his vote from a message

Adding vote is handled correctly (the added reaction is just removed, the user is DMed).

Solution could be: prevent removing the reaction (if it's possible).

[nisnevich]

Fixed.