[SPARK-8581] [SPARK-8584] Simplify checkpointing code + better error message

This patch rewrites the old checkpointing code in a way that is easier to understand. It also adds a guard against an invalid specification of checkpoint directory to provide a clearer error message. Most of the changes here are relatively minor.

Author: Andrew Or <andrew@databricks.com>

Closes #6968 from andrewor14/checkpoint-cleanup and squashes the following commits:

4ef8263 [Andrew Or] Use global synchronized instead
6f6fd84 [Andrew Or] Merge branch 'master' of github.com:apache/spark into checkpoint-cleanup
b1437ad [Andrew Or] Warn instead of throw
5484293 [Andrew Or] Merge branch 'master' of github.com:apache/spark into checkpoint-cleanup
7fb4af5 [Andrew Or] Guard against bad settings of checkpoint directory
691da98 [Andrew Or] Simplify checkpoint code / code style / comments