Logging related to WikiExport cleanup and changes in T203424

Logging related to WikiExport cleanup and changes in T203424

T203424 (Replace the WikiExporter backup dump streaming mode with batched queries)
removed some outdated database usage techniques.  While making these
changes, we saw some potential cleanup opportunities related
to the $history parameter to WikiExporter.  Additional logging will
inform whether we can do the additional cleanup.  If so, it can be
performed under a separate task.

Bug: T203424
Change-Id: Ic4cef23520795665c89e03f325b891346d1340a9