Reassess partitioning queries by tablet location when using scan servers

[keith-turner]

Execution of queries may use PushdownFunction which seems to partition queries based on tablet locations on tablet servers. When using scan servers these locations are not very meaningful for query planning. Looking forward to Accumulo 4 where tables can load tablets into tablet servers on demand attempting to obtain the location of an ondemand tablet would break the current PushdownFunction code.

What changes should be made to this code to make it work well scan servers and eventually on demand tablets?