make QNetworkDiskCache cache files shareable

Currently one cannot share the same cache among multiple processes.
This is a relative problem for end applications, but can be a problem for Qt components such as QtLocation, that may need to have a cache "per component", and more than one application using that component might run at the same time.

The simplest solution would probably be a sister class not based on files but on a sqlite db, that would allow safe concurrent access to the resources.