Make Future implementation on Connection unconditional on executor being Send + Sync.

[mstyura]

This PR is created to raise the question if Future implementation on Connection is mandatory to request Executor to be Send + Sync.
Question initially raised here https://discord.com/channels/500028886025895936/670880858630258689/1247563996068970557.

[mstyura]

Interesting, CI does rustfmt --edition 2021 $(git ls-files '*.rs') and it formats code, while cargo fmt does not.

[seanmonstar]

I think this looks good, any reason it was marked a draft?

[mstyura]

It was initially supposed to raise a question, that's why I've marked it as draft. But as long as you are ok with changes I will unmask as draft and fix issues if any according to review