Feature delay argument for iterator

[philipperenzen]

This feature adds the "delay" argument for the iterator configuration, which will diminish the frequency for endpoint requests for the iterator.

[mightymax]

@LaurensRietveld or @philipperenzen was this delay implemented to be "more kind" to SPARQL endpoints or is it to prevent a queue of incoming triples becoming too large to process for LD Workbench itself? We would like to remove this extra configuration option to keep it as simple as possible.