Inefficient HttpClient in SPARQLClosedWorldIndividualRetriever

[TortugaAttack]

Move the ClosableHttpClient out of the SPARQLClosedWorldIndividualRetriever.createRequest and use it everytime to save time.
Also add a finally block consuming the entity, to assure that the request can be closed

[MichaelRoeder]

Documentation and some examples can be found at http://hc.apache.org/httpcomponents-client-5.1.x/quickstart.html (other client versions can be found on the left side of the page).