[SPARK-6860][Streaming][WebUI] Fix the possible inconsistency of StreamingPage

Because `StreamingPage.render` doesn't hold the `listener` lock when generating the content, the different parts of content may have some inconsistent values if `listener` updates its status at the same time. And it will confuse people.

This PR added `listener.synchronized` to make sure we have a consistent view of StreamingJobProgressListener when creating the content.

Author: zsxwing <zsxwing@gmail.com>

Closes #5470 from zsxwing/SPARK-6860 and squashes the following commits:

cec6f92 [zsxwing] Add missing 'synchronized' in StreamingJobProgressListener
7182498 [zsxwing] Add synchronized to make sure we have a consistent view of StreamingJobProgressListener when creating the content