handle client side caching

[eroux]

We should have a few caching mechanisms:

• [ ] a general mechanism for all data requests (everything coming from fuseki), where clients can cache the result during maybe one day (this should be an option of lds-pdi)
• [ ] a specific mechanism for everything related to the ontology (the context.jsonld and various enpoints where the ontology is available), where we can record the date of last update, and use that in the cache headers (example)
• [ ] add cache headers to Access-Control-Allow-Headers and possibly Access-Control-Expose-Headers headers, I guess we need at least Keep-Alive,User-Agent,If-Modified-Since,Cache-Control,Content-Type,Accept-Encoding

[MarcAgate]

All done as of commit b007cf8

• All data responses use custom @JerseyCacheControl() annotation having max-age=172800 as default (2 days).

• All ontology related responses use the Etag mechanism (each url having its own Etag) and expose Last-Modified header

[eroux]

looks good, thanks!