[SPARK-13450][SQL] External spilling when join a lot of rows with the same key

[shenh062326]

SortMergeJoin use a ArrayBuffer[InternalRow] to store bufferedMatches, if the join have a lot of rows with the same key, it will throw OutOfMemoryError.
Add a ExternalAppendOnlyArrayBuffer to store bufferedMatches instand of ArrayBuffer.

[SparkQA]

Test build #52032 has finished for PR 11386 at commit b0c00f4.

• This patch fails RAT tests.
• This patch merges cleanly.
• This patch adds the following public classes (experimental):
  • class ExternalAppendOnlyArrayBuffer[T: ClassTag](

[rxin]

Thanks for the pull request. I'm going through a list of pull requests to cut them down since the sheer number is breaking some of the tooling we have. Due to lack of activity on this pull request, I'm going to push a commit to close it. Feel free to reopen it or create a new one. We can also continue the discussion on the JIRA ticket.