RdfDataSource: Optimize repeated wrapping with algebra transforms.

[Aklakan]

Consecutive wrappings of an RdfDataSource with algebra transformations will call Algebra.compile and OpAsQuery for each transform.
The wrap methods should automatically optimize the built RdfDataSource such that subsequent algebra transforms are carried out on the Op-level - i.e. without converting back and forth to the syntax form.

RdfDataSource dataSource = () -> RDFConnection.connect(...);
dataSource = RdfDataSources.wrapWithOpTransform(base, opTransform1);
dataSource = RdfDataSources.wrapWithOpTransform(base, opTransform2);