Add resourceUriTargetCache to SMW\SPARQLStore\RedirectLookup

XDebug profiling uncovered that repeated requests are made to the same
resource uri and to improve performance an in-memory cache is added to
avoid unnecessary DB lookup requests for already known uri's.
Add dataItemExportCache to SMW\SPARQLStore\TurtleTriplesBuilder

Improves performance especially when subobjects are involved.
Merge pull request #487 from SemanticMediaWiki/sparql-redirectlookup-cache

Add resourceUriTargetCache to SMW\SPARQLStore\RedirectLookup