SPARK-4743 - Use SparkEnv.serializer instead of closureSerializer in aggregateByKey and foldByKey

Author: Ivan Vergiliev <ivan@leanplum.com>

Closes #3605 from IvanVergiliev/change-serializer and squashes the following commits:

a49b7cf [Ivan Vergiliev] Use serializer instead of closureSerializer in aggregate/foldByKey.