Building with Scala 2.11 results in the warning trait SynchronizedBuffer in package mutable is deprecated: Synchronization via traits is deprecated as it is inherently unreliable. Consider java.util.concurrent.ConcurrentLinkedQueue as an alternative - we already use ConcurrentLinkedQueue elsewhere so lets replace it.

Some notes about how behaviour is different for reviewers:
The Seq from a SynchronizedBuffer that was implicitly converted would continue to receive updates - however when we do the same conversion explicitly on the ConcurrentLinkedQueue this isn't the case. Hence changing some of the (internal & test) APIs to pass an Iterable. toSeq is safe to use if there are no more updates.

Author: Holden Karau <holden@us.ibm.com>
Author: tedyu <yuzhihong@gmail.com>

Closes #11067 from holdenk/SPARK-13165-replace-deprecated-synchronizedBuffer-in-streaming.