[SPARK-6046] [core] Reorganize deprecated config support in SparkConf.

This change tries to follow the chosen way for handling deprecated
configs in SparkConf: all values (old and new) are kept in the conf
object, and newer names take precedence over older ones when
retrieving the value.

Warnings are logged when config options are set, which generally happens
on the driver node (where the logs are most visible).

Author: Marcelo Vanzin <vanzin@cloudera.com>

Closes #5514 from vanzin/SPARK-6046 and squashes the following commits:

9371529 [Marcelo Vanzin] Avoid math.
6cf3f11 [Marcelo Vanzin] Review feedback.
2445d48 [Marcelo Vanzin] Fix (and cleanup) update interval initialization.
b6824be [Marcelo Vanzin] Clean up the other deprecated config use also.
ab20351 [Marcelo Vanzin] Update FsHistoryProvider to only retrieve new config key.
2c93209 [Marcelo Vanzin] [SPARK-6046] [core] Reorganize deprecated config support in SparkConf.