Checksum/sanity checks for cache files

[pfitzseb]

@fredrikekre ran into an OOM while deserializing a cache file, which got corrupted somehow. Would be good to detect that during deserialization and try to recover (or just delete corrupt caches).

[fredrikekre]

The file: v3.3.1_9ba33637b24341aba594a2783a502760aa0bff04.jstore.gz