Switch from slog to tracing

[data-sync-user]

tracing has become the standard logging library to use in async/distributed Rust applications. We should move away from using slog in favor of tracing. Merino makes use of tracing in a way that relies on mozlog Actix middleware to format the logs as mozlogs. We don’t rely heavily on logging to get information about our service at runtime (we use metrics instead since they’re cheaper), so we probably don’t need to investigate making use of spans or any other more involved features. For now, replacing the slog macro calls with tracing macros should be sufficient.

┆Issue is synchronized with this Jira Task