[RFC] Avoid calling await while holding a lock

[imobachgs]

Clippy reports that in our code we call await while holding a lock. There are two possible solutions:

• Make sure than the Mutex is drop before calling await (this).
• Use an async-aware Mutex implementation (Use an async-aware Mutex implementation #664).

This PR implements the first solution. I have decided to use an scope instead of calling drop because of rust-lang/rust-clippy#6446.

[coveralls]

coverage: 72.371% (-0.01%) from 72.383% when pulling 63cbe36 on imobachgs:fix-await-holding-lock into 2e11beb on openSUSE:master.

[imobachgs]

Finally went with #664.