Cache size logging is very spammy

Summary

The UI shows us every time the cache size lock file is written too. This behaviour occurs by default and seems both spammy and unnecessary to me.

What is more, is the fact that the only information these logs give you is whether we successfully wrote to the lock file or not (which we almost (if not) always do), and how long this took (usually < 1s).

Example log file below:

$ cat /home/jennis/.cache/buildstream/logs/cache_size/cache_size.39249.log 
[--:--:--] START  : cache_size
[00:00:00] SUCCESS: cache_size

And this is what it looks like to the user:

Personally, I don't think that this should be default behaviour. Perhaps we just simply avoid printing this information?