Replace slog with log once log supports structured logging

[tomaka]

The entirety of Substrate uses the log crate, except for the telemetry which uses slog because of structured logging.

The log is currently getting support for structured logging:
rust-lang/log#149
rust-lang/log#328

Once this is done, we can switch out from slog.

[yoshuawuyts]

https://github.com/yoshuawuyts/kv-log-macro now works, which coupled with the kv_unstable macro should probably cover most of what you might want from a logging solution (:

[gnunicorn]

We're using slog just to have a global logger and to serialize json, we could replace it with serde_json and lazy_static

[tomaka]

If someone tackles this issue, please add me as a reviewer.

[bkchr]

Telemetry isn't using slog anymore.