Support diesel_logger outside of unit tests

[pjenvey]

Logging diesel's queries via diesel_logger can be useful outside of unit tests. Currently diesel_logger's LoggingConnection is hardcoded to #[cfg(test)].

We could instead make this a setting to enable it (e.g. SYNC_DATABASE_DIESEL_LOGGER=true) or just always enable it if it doesn't cost much in overhead.

This feature could also use a couple more things:

1. consider changing the log crate's max value in debug builds to max_level_debug (in Cargo.toml). Though this change might add a lot of extra log spew during development (this needs some investigation)?
2. docs on how to use it (maybe unnecessary or very little needed if Initial Rust project with file structure layout #1 is fixed)

[jrconlin]

One possible bit of annoyance is that there is no way (outside of using LoggingConnection and monitoring the output) to capture the composed SQL statement from diesel. If we could, we could use something like tokio-trace to selectively record potentially interesting statements.