Add RebuildConceptCacheMaintenanceRegressionTest
Fix SMWSQLStore3Writers::prepareConceptTableInserts

A missing id caused table smw_fpt_conc on MySQL to fill up with ('','','0','-1','-1',NULL,NULL) entries
while SQLite would fail immediately.
Merge pull request #272 from SemanticMediaWiki/regression-maintenance-concept

Add RebuildConceptCacheMaintenanceRegressionTest