Source selector cache initialization

[Antonis Troumpoukis]

Prefix source selection initializes its cache only once (when is called for the first time), and this can lead to errors.
Consider two sources, ex:t1 appears in the 1st source, ex:t2 appears in the second and rdf:type and geo:hasGeometry in both.
Example: If i run

SELECT * WHERE { ?s rdf:type ex:t1 . ?s geo:hasGeometry ?g . }

it returns results.
But if i first run

SELECT * WHERE { ?s rdf:type ex:t2. ?s geo:hasGeometry ?g . }

and then

SELECT * WHERE { ?s rdf:type ex:t1 . ?s geo:hasGeometry ?g . }

the second query does not return results.