Upgrade to diesel-async?

[smklein]

We are using https://github.com/oxidecomputer/async-bb8-diesel extensively throughout Nexus, so that we can asynchronously issue diesel operations.

An upgrade to https://crates.io/crates/diesel-async would provide the following benefits:

1. First-class support. async-bb8-diesel is maintained by us, by swapping between tokio threads, but diesel-async is supported by diesel devs.
2. Pipelined requests. diesel-async uses tokio_postgres, which supports pipelining: https://docs.rs/tokio-postgres/latest/tokio_postgres/index.html#pipelining
3. ... speaking of which, tokio_postgres is a rust client library, which might help reduce our reliance on the libpq library.