Preparations for async multi connection

[jusax23]

This PR makes changes needed for an AsyncMultiConnection:

• adds AsyncMultiConnectionHelper
  • implemented for AsyncMysqlConnection
  • implemented for PgAsyncMetadataLookup
  • implemented for SyncConnectionWrapper
• changes of lifetimes of SyncConnectionWrapper

prior discussion: #108

[weiznich]

Thanks for opening this PR. The changes look good to me, although I would like to wait with merging them until the second part (changes to the derive) is also done.

[jusax23]

Hi, I've pushed a new commit to this to pr, to add expand_pool. I've added this to lib.rs, I hope thats okay.

[weiznich]

That's fine

This still needs to include an adjusted copy of https://github.com/diesel-rs/diesel/blob/main/diesel_derives/src/multiconnection.rs to demonstrate that the derive works

[BlackDex]

This PR might be useful for Vaultwarden to get rid of the MySQL/MariaDB issues.
Not sure what kind of other impact it might have regarding secure connections or specific authentication protocols though.

[jusax23]

Hi @weiznich , I've pushed a modified version of the sync test. It runs sucessful on my machine, when i change the diesel dependency to a local copy of main. So this pr can only be merged after a supported diesel version is released, or with the added module in tests/lib.rs commented out.