[SPARK-3369][CORE][STREAMING] Java mapPartitions Iterator->Iterable is inconsistent with Scala's Iterator->Iterator

Fix Java function API methods for flatMap and mapPartitions to require producing only an Iterator, not Iterable. Also fix DStream.flatMap to require a function producing TraversableOnce only, not Traversable.

CC rxin pwendell for API change; tdas since it also touches streaming.

Author: Sean Owen <sowen@cloudera.com>

Closes #10413 from srowen/SPARK-3369.