Support filtering statements by "best rank" rule in RDF export

[mkroetzsch]

The query service for Wikidata will restrict parts of its dataset to statements that have the highest possible non-deprecated rank for their property and item. We should support this too. Detailed rules for when to export a statement with property P1234 on an item Q1234:

• If Q1234 has a P1234-statement of preferred rank, export only P1234-statements of preferred rank.
• If Q1234 has no P1234-statement of preferred rank, then export all P1234-statements of normal rank.
• Never export any statement of deprecated rank.

This behaviour should be configurable as a task in the RDF exporter.