mediawiki_messages reuses QueryGenerator

When mediawiki_messages is called with a specific set of keys requested,
it inefficiently and incorrectly iterates the generator for each key.
This breaks a fundamental assumption of a 'generator' - that it can not
be re-used.  It is assuming that QueryGenerator was able to fetch all
the requested messages without continuation, as only the last fetch
will exist in QueryGenerator.data after the first iteration is complete.

Change-Id: Ia50ec73d33caa548227dfcbd52119117720787dc