Migrate to Diesel 2.0

The bulk work is done, but the following would have to be double checked:

• Database migrations are run inside a transaction - that can't roll back changes in tables, right?
• Is Error::RollbackTransaction used correctly?
• Using multiple batch_execute() calls in a row feels dumb, but results in better error messages...

But: it doesn't compile:

error[E0596]: cannot borrow data in a dereference of `ReentrantMutexGuard<'_, parking_lot::RawMutex, RawThreadId, diesel::SqliteConnection>` as mutable
  --> whisperfish/src/store/protocol_store.rs:68:43
   |
68 |             prekeys.select(max(id)).first(&mut *db).expect("db")
   |                                           ^^^^^^^^ cannot borrow as mutable
   |

And 9 other similar errors. I have literally no idea what causes that. The same error comes with up-to-date cargo clippy run, and trying to compile inside sfdk.

Fixes #227 (closed)