Revert part of cf8de026f5f743c4c99867d549141c57bf436cd0

tokio-rs/tracing pulls in a lot more dependencies that log, also potentially impacting negatively the build time of the crate. While this was an acceptable choice for a single user of the crate, which was what I had in mind when I made this commit in the first place, it is not perhaps not a sensible choice for a project that aims at gaining tractiong and seeks to provide a simple, safe and abstract wrapper for libnftnl. Hence the move back to the log crate.