[SPARK-5771] Master UI inconsistently displays application cores

If the user calls `sc.stop()`, then the number of cores under "Completed Applications" will be 0. If the user does not call `sc.stop()`, then the number of cores will be however many cores were being used before the application exited. This PR makes both cases have the behavior of the latter.

Note that there have been a series of PR that attempted to fix this. For the full discussion, please refer to #4841. The unregister event is necessary because of a subtle race condition explained in that PR.

Tested this locally with and without calling `sc.stop()`.

Author: Andrew Or <andrew@databricks.com>

Closes #5177 from andrewor14/master-ui-cores and squashes the following commits:

62449d1 [Andrew Or] Freeze application state before finishing it