Creating a list of scan tokens should retrieve tablets in larger batches

In a test on a 200-node cluster with 40 concurrent query streams, we found that the Impala planner was sometimes taking minutes to fetch the list of scan tokens. The tables in the query had several thousand tablets, so with the default batch size of 10 tablets per GetTableLocations RPC, the planning required hundreds of round trips, each of which had some chance of getting bumped from the queue due to backpressure, etc.

A local hack to change the batching to 1000 tablets per RPC reduced the planning times down to sub-second.