GEOMESA-311 Handle 'attr1 and attr2' filters

If multiple attribute filters are indexable, use the first one
In the future, a query optimizer will make more informed decision
Can indexFilter be None?  I'm pretty sure the answer is 'yes'.  (Meaning that the .get is a Problem.)

I'm a little unclear on what this function is trying to do.  I'd say comment it or at least rename it more suggestively.