Avoid unwanted writing of measurements

Only write the performance of copy-operations if the corresponding
files promise has a measurement_class set.

See redmine #6523
Don't write bundle performance to disk unnecessarily

Timing measurements for bundles is a help for the policy writer,
and not particularly relevant in automated CFEngine runs. So skip
the writing unless the log level is at least INFO.
Merge pull request #1981 from vohi/redmine/6523-performancedb

Redmine/6523: don't hit performance database unnecessarily