[SPARK-8127] [STREAMING] [KAFKA] KafkaRDD optimize count() take() isEmpty()

…ed KafkaRDD methods.  Possible fix for [SPARK-7122], but probably a worthwhile optimization regardless.

Author: cody koeninger <cody@koeninger.org>

Closes #6632 from koeninger/kafka-rdd-count and squashes the following commits:

321340d [cody koeninger] [SPARK-8127][Streaming][Kafka] additional test of ordering of take()
5a05d0f [cody koeninger] [SPARK-8127][Streaming][Kafka] additional test of isEmpty
f68bd32 [cody koeninger] [Streaming][Kafka][SPARK-8127] code cleanup
9555b73 [cody koeninger] Merge branch 'master' into kafka-rdd-count
253031d [cody koeninger] [Streaming][Kafka][SPARK-8127] mima exclusion for change to private method
8974b9e [cody koeninger] [Streaming][Kafka][SPARK-8127] check offset ranges before constructing KafkaRDD
c3768c5 [cody koeninger] [Streaming][Kafka] Take advantage of offset range info for size-related KafkaRDD methods.  Possible fix for [SPARK-7122], but probably a worthwhile optimization regardless.