RelatedSearchQuerySet: move class globals to instance properties
RelatedSearchQuerySet: move class globals to instance properties

This caused obvious failures in the test suite and presumably
elsewhere when multiple RelatedSearchQuerySet instances were in use

jannon/django-haystack,eventials/django-haystack,barseghyanartur/django-haystack,ruimashita/django-haystack,fisle/django-haystack,celerityweb/django-haystack,antonyr/django-haystack,eventials/django-haystack,kuanyui/django-haystack,sgaist/django-haystack,comandrei/django-haystack,eventials/django-haystack,Stupeflix/django-haystack,ruimashita/django-haystack,fisle/django-haystack,celerityweb/django-haystack,sgaist/django-haystack,celerityweb/django-haystack,antonyr/django-haystack,ruimashita/django-haystack,zeehio/django-haystack,sgaist/django-haystack,blancltd/django-haystack,jannon/django-haystack,django-searchstack/django-searchstack,blancltd/django-haystack,Stupeflix/django-haystack,kybi/django-haystack,comandrei/django-haystack,barseghyanartur/django-haystack,vitalyvolkov/django-haystack,blancltd/django-haystack,Stupeflix/django-haystack,streeter/django-haystack,cyanut/django-haystack,barseghyanartur/django-haystack,streeter/django-haystack,django-searchstack/django-searchstack,cyanut/django-haystack,antonyr/django-haystack,kuanyui/django-haystack,zeehio/django-haystack,streeter/django-haystack,kybi/django-haystack,kuanyui/django-haystack,comandrei/django-haystack,fisle/django-haystack,django-searchstack/django-searchstack,jannon/django-haystack,vitalyvolkov/django-haystack,vitalyvolkov/django-haystack,elishowk/django-haystack,zeehio/django-haystack,elishowk/django-haystack,cyanut/django-haystack,elishowk/django-haystack