Add build option to keep cache on resolver teardown

[WD-stefaang]

The destructor of the SQL context deletes all files from the cache.
Would it make sense to have an option to keep the local cache alive?
The fetch_asset function would then have to check the local cache.

It could be a bit tricky to compare the date modified between local files and the database version though.

An alternative is to serialize the cache on teardown and attempt to load it on init.

[sirpalee]

I would rather make it an option via environment variables. That makes switching between behaviors way easier, and no need to distribute two different binaries.

Other than this, to make this happen we need to name the local files so they are easier to look up, and make the local cache directory configurable.

[WD-stefaang]

Indeed, environment vars are the best way to go.

If you serialize the cache on teardown and try to load it on init, you wouldn't need to make the filenames easier. On the other hand... I'm not using generate_name but a configurable cache dir with simple filenames, unfortunately this prevents the use of multiple versions of the same asset in a single stage (as it gets stored to the same location).