Cache log is too verbose

Fix of #278 (closed)

The unsupported_cache log should only be displayed when the worker tries to use the cache self.use_cache. Whether the worker is running in read-only mode is unrelated for this log.

Refs https://redmine.teklia.com/issues/8935

changed milestone to %0.4.0

added P3 Quick Win labels

assigned to @ebardou

mentioned in merge request !603 (merged)

closed with merge request !603 (merged)