There are multiple issues with translating on set outlined in the JIRA.

This PR reverts the translation logic added to `SparkConf`. In the future, after the 1.3.0 release we will figure out a way to reorganize the internal structure more elegantly. For now, let's preserve the existing semantics of `SparkConf` since it's a public interface. Unfortunately this means duplicating some code for now, but this is all internal and we can always clean it up later.

Author: Andrew Or <andrew@databricks.com>

Closes #4799 from andrewor14/conf-set-translate and squashes the following commits:

11c525b [Andrew Or] Move warning to driver
10e77b5 [Andrew Or] Add documentation for deprecation precedence
a369cb1 [Andrew Or] Merge branch 'master' of github.com:apache/spark into conf-set-translate
c26a9e3 [Andrew Or] Revert all translate logic in SparkConf
fef6c9c [Andrew Or] Restore deprecation logic for spark.executor.userClassPathFirst
94b4dfa [Andrew Or] Translate on get, not set