[SPARK-6014] [core] Revamp Spark shutdown hooks, fix shutdown races.

This change adds some new utility code to handle shutdown hooks in
Spark. The main goal is to take advantage of Hadoop 2.x's API for
shutdown hooks, which allows Spark to register a hook that will
run before the one that cleans up HDFS clients, and thus avoids
some races that would cause exceptions to show up and other issues
such as failure to properly close event logs.

Unfortunately, Hadoop 1.x does not have such APIs, so in that case
correctness is still left to chance.

Author: Marcelo Vanzin <vanzin@cloudera.com>

Closes #5560 from vanzin/SPARK-6014 and squashes the following commits:

edfafb1 [Marcelo Vanzin] Better scaladoc.
fcaeedd [Marcelo Vanzin] Merge branch 'master' into SPARK-6014
e7039dc [Marcelo Vanzin] [SPARK-6014] [core] Revamp Spark shutdown hooks, fix shutdown races.

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