Quiet dbg! calls

[data-sync-user]

(From the original https://github.com/mozilla-services/syncstorage-rs/issues/1101)

We've added some recent dbg! calls that output to our deployed logs, they should likely be converted to trace! (or something else) -- so they don't output to our logs.

┆Issue is synchronized with this Jira Task

[jrconlin]

sigh

Let's also add --deny=clippy::dbg_macro to the clippy checks.