bump Diesel to v2

[Asone]

[Asone]

A migration guide is available here

[Asone]

I began to bump the dependency. However rocket_db_sync_pool crate doesn't handle - yet - diesel 2.0.0.

A fix has been committed in master branch of rocket, however no release has shipped the mentioned commit yet.

This issue will be on stale while awaiting for a new release providing such fix.

More details are available here