Improvements to the snapshot disk cache

[mechanik-daniel]

Sometimes, snapshots should be re-generated:

• If the snapshot generator function has changed
• If any of the dependencies have changed

And sometimes, a snapshot that has already been generated could be re-used but it's not:

• If the username running FUME changes, so does the path to the resource, hence the hash key in the cache is different
• If a snapshots folder content is copied between installations in order to save loading/parsing times and loads

Improvements may include:

• Keys generated based on actual content instead of file system paths
• Use the FUME engine version even better - snapshot generator version - as part of the hash
• Include dependencies content and not just the direct resource content