[SPARK-10003] Improve readability of DAGScheduler

Note: this is not intended to be in Spark 1.5!

This patch rewrites some code in the `DAGScheduler` to make it more readable. In particular
- there were blocks of code that are unnecessary and removed for simplicity
- there were abstractions that are unnecessary and made the code hard to navigate
- other minor changes

Author: Andrew Or <andrew@databricks.com>

Closes #8217 from andrewor14/dag-scheduler-readability and squashes the following commits:

57abca3 [Andrew Or] Move comment back into if case
574fb1e [Andrew Or] Merge branch 'master' of github.com:apache/spark into dag-scheduler-readability
64a9ed2 [Andrew Or] Remove unnecessary code + minor code rewrites