Extend simple query cache to SSD

[juliangruendner]

The Cache of the Requestor of FLARE should be extended to allow caching on disk rather than only in memory as currently implemented using the Caffeine Cache.

https://github.com/rwth-imi/flare-query/blob/develop/requestor/src/main/java/de/rwth/imi/flare/requestor/FhirRequestor.java

It should be investigated how either Caffeine can be extended to write to disk or how a different tool such as MapDB can be used - see ben-manes/caffeine#168

The Cache should Cache exactly the same key/values as it currently does:
key: SearchRequestString, value:

As part of this issue the new cache should be analysed for performance