[feature] Resume: print cache invalidation reason to console

[ewels]

Idea for making broken -resume caches less frustrating: get Nextflow to identify the change that caused an invalidated cache, then print to the console. Give immediate feedback, rather than forcing the end user to dig into hashes etc to try to figure this out.

Related to #4214

[bentsherman]

See also: #4369 (comment)

I originally imagined having a nextflow diff command where you specify two runs and it compares them. Doing this during the resumed run is much better UX but requires you to determine which task to compare from the previous run. Probably the task the same name e.g. FASTQC (1) and from the same session id..

If such a task isn't found then we must fall back to a simple message like "this task looks new" which isn't as helpful but better than nothing.