Add ESSearchGetKeysByScore(), which works similiar to searchAndReturnAssociationKeys() but returns score as well

1 - ESSearchGetKeysByScore() and searchAndReturnAssociationKeys() are very similiar.  We can probably delete the second one or
    make the behavior configurable.  I did not want to do it yet because I'm not ready to convert all uses of
    searchAndReturnAssociationKeys() to use the new format.  It's also used by any calls to plain old search() and I'm not sure
    what calls that.

2 - I named it with "ES" in front because once you're in a model, using ->searchAndReturnAssociationKeys() did not make it obvious
    that function was using elastic search.