SPARK-2043: ExternalAppendOnlyMap doesn't always find matching keys

The current implementation reads one key with the next hash code as it finishes reading the keys with the current hash code, which may cause it to miss some matches of the next key. This can cause operations like join to give the wrong result when reduce tasks spill to disk and there are hash collisions, as values won't be matched together. This PR fixes it by not reading in that next key, using a peeking iterator instead.

Author: Matei Zaharia <matei@databricks.com>

Closes #986 from mateiz/spark-2043 and squashes the following commits:

0959514 [Matei Zaharia] Added unit test for having many hash collisions
892debb [Matei Zaharia] SPARK-2043: don't read a key with the next hash code in ExternalAppendOnlyMap, instead use a buffered iterator to only read values with the current hash code.

(cherry picked from commit b45c13e7d798f97b92f1a6329528191b8d779c4f)
Signed-off-by: Matei Zaharia <matei@databricks.com>