benchmark pkg: allow TrecContentSource not to change the docname

TrecContentSource currently appends 'iteration number' to the docname field.
Example: if the original docname is DOC0001 then it will be indexed as DOC0001_0

this presents a problem for relevance testing, because when judging results, the expected docname will never be present.
This patch adds an option to disable this behavior, defaulting to the existing behavior (which is to append the iteration number).