mediawiki_messages reuses QueryGenerator
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

emijrp/pywikibot-core,Darkdadaah/pywikibot-core,wikimedia/pywikibot-core,magul/pywikibot-core,wikimedia/pywikibot-core,Darkdadaah/pywikibot-core,happy5214/pywikibot-core,npdoty/pywikibot,happy5214/pywikibot-core,h4ck3rm1k3/pywikibot-core,PersianWikipedia/pywikibot-core,magul/pywikibot-core,hasteur/g13bot_tools_new,h4ck3rm1k3/pywikibot-core,jayvdb/pywikibot-core,valhallasw/pywikibot-core,xZise/pywikibot-core,smalyshev/pywikibot-core,VcamX/pywikibot-core,hasteur/g13bot_tools_new,npdoty/pywikibot,TridevGuha/pywikibot-core,darthbhyrava/pywikibot-local,trishnaguha/pywikibot-core,icyflame/batman,jayvdb/pywikibot-core,hasteur/g13bot_tools_new