[SPARK-8372] History server shows incorrect information for application not started

The history server may show an incorrect App ID for an incomplete application like <App ID>.inprogress. This app info will never disappear even after the app is completed.
![incorrectappinfo](https://cloud.githubusercontent.com/assets/9278199/8156147/2a10fdbe-137d-11e5-9620-c5b61d93e3c1.png)

The cause of the issue is that a log path name is used as the app id when app id cannot be got during replay.

Author: Carson Wang <carson.wang@intel.com>

Closes #6827 from carsonwang/SPARK-8372 and squashes the following commits:

cdbb089 [Carson Wang] Fix code style
3e46b35 [Carson Wang] Update code style
90f5dde [Carson Wang] Add a unit test
d8c9cd0 [Carson Wang] Replaying events only return information when app is started