Add KUDU_REPLICA_SELECTION config and query option

Sometimes it is useful to target queries at the LEADER_ONLY instead of the default CLOSEST_REPLICA. We should expose this option in Impala similar to the way KUDU_READ_MODE is exposed:
https://impala.apache.org/docs/build/html/topics/impala_kudu_read_mode.html

Currently there is a test only flag, FLAGS_pick_only_leaders_for_tests, that somewhat does this:
https://github.com/apache/impala/blob/da2999afd9ddc45d35141649d17db507e03ee9bf/be/src/exec/kudu-scanner.cc#L195-L198