Review database pool abstraction

[aumetra]

Review database pool abstraction

[aumetra]

New API idea:

1. Rename .with_connection to .pipeline to change the semantics by naming
2. Implement the required diesel-async traits for the pool which internally just borrow a connection and execute it on the connection (traits being AsyncConnection, SimpleAsyncConnection, and UpdateAndFetchResults)

---

This would allow pipelining the same way as before:

pool.pipeline(|conn| async move {
       [query]
   }
   .scoped()
).await?;

and would simplify single queries that can't be pipelined:

users::table.find(user_id).load(&mut pool).await?

[aumetra]

One of the tradeoffs here is one more layer of boxing futures. Since diesel-async uses async-trait for various traits, we would have to box again meaning we poll a boxed future inside a boxed future, missing some inline opportunities.

Won't break the world and we will still be faster by multiple magnitudes than other implementations, just something to keep in mind for now.