Properly address sp_tracing and log usage

[dmitrylavrenov]

As a follow up of #1325 - when would we need to use tracing (and when not - and use just log)

Please, check the details here - #1325 (comment)
Also, see #1326

Reading up on the comments in that PR - it seems like tracing is not used for logging in the general purpose, but for something else that is kind of very specific and that I didn't anticipate they'd design like that. In particular - there is a specific feature for block re-execution with tracing. Who would want that and why is a mystery to me as of now - but at least it is now clear there is a distinction in the purpose of the regular logging infrastructure and sp_tracing logging. It is clear that the difference is beyond them just being two logging frontends - as they seem to be targeting different backends - so we have to learn and figure out when to use which.

To be more explicit here - I clearly was wrong about always using tracing. I think that, by default, we'd actually want to use frame_support::log APIs.

We also must figure out what they use tracing for.