[SPARK-17843][WEB UI] Indicate event logs pending for processing on history server UI

## What changes were proposed in this pull request?

History Server UI's application listing to display information on currently under process event logs so a user knows that pending this processing an application may not list on the UI.

When there are no event logs under process, the application list page has a "Last Updated" date-time at the top indicating the date-time of the last _completed_ scan of the event logs. The value is displayed to the user in his/her local time zone.
## How was this patch tested?

All unit tests pass. Particularly all the suites under org.apache.spark.deploy.history.\* were run to test changes.
- Very first startup - Pending logs - no logs processed yet:

<img width="1280" alt="screen shot 2016-10-24 at 3 07 04 pm" src="https://cloud.githubusercontent.com/assets/12079825/19640981/b8d2a96a-99fc-11e6-9b1f-2d736fe90e48.png">
- Very first startup - Pending logs - some logs processed:

<img width="1280" alt="screen shot 2016-10-24 at 3 18 42 pm" src="https://cloud.githubusercontent.com/assets/12079825/19641087/3f8e3bae-99fd-11e6-9ef1-e0e70d71d8ef.png">
- Last updated - No currently pending logs:

<img width="1280" alt="screen shot 2016-10-17 at 8 34 37 pm" src="https://cloud.githubusercontent.com/assets/12079825/19443100/4d13946c-94a9-11e6-8ee2-c442729bb206.png">
- Last updated - With some currently pending logs:

<img width="1280" alt="screen shot 2016-10-24 at 3 09 31 pm" src="https://cloud.githubusercontent.com/assets/12079825/19640903/7323ba3a-99fc-11e6-8359-6a45753dbb28.png">
- No applications found and No currently pending logs:

<img width="1280" alt="screen shot 2016-10-24 at 3 24 26 pm" src="https://cloud.githubusercontent.com/assets/12079825/19641364/03a2cb04-99fe-11e6-87d6-d09587fc6201.png">

Author: Vinayak <vijoshi5@in.ibm.com>

Closes #15410 from vijoshi/SAAS-608_master.