Mix up of terms in trident sources in VLog

[mkroetzsch]

Under some conditions, trident database sources return mixed-up results in VLog. It looks as if the trident content is used with the dictionary generated for another RDF source. To reproduce, generate an EDB configuration that loads an RDF source before a trident source. Then query the trident source.

[CerielJacobs]

For now, this has been resolved by allowing only one trident source and making sure it comes first. Not really a fix, so I'll leave this issue open.