No longer use the log crate in the light client

[tomaka]

Close #756

This PR removes usage of the log crate from the light client and instead adds a new function on the PlatformRef trait.
The DefaultPlatform uses the log crate, which means that the behavior actually doesn't change compared to before.

This PR is however a breaking change as it modifies the PlatformRef trait. See the changes to light-base/src/platform/default.rs to see how to migrate. cc @skunert @ermalkaleci @lexnv as a heads up.

I'm doing this for two reasons:

• Having key-values logging is better than just a plain message for a variety of reasons, notably being able to parse log messages more easily for indexing. The kv_unstable feature of the log crate has been there for, I don't know, years? and still is nowhere near stabilization.
• In case where there are multiple smoldot light clients running in parallel in the same process, which is useful in testing situations, it is useful to be able to add a prefix before the target of every log message. This is simply impossible to do with the log crate (there's a reason why global variables are considered as a bad thing).

An alternative could have been to use the tracing crate, but I generally really dislike the tracing crate due to being over-engineered and confusing. It shouldn't take ten of hours of learning in order to print a log message.

As part of this refactoring, I've rewritten all the log messages. Since logs are mostly for bug reports, this should be completely fine. I think and I hope that nobody was parsing log message programatically.