[SPARK-6752] [STREAMING] [REOPENED] Allow StreamingContext to be recreated from checkpoint and existing SparkContext

Original PR #5428 got reverted due to issues between MutableBoolean and Hadoop 1.0.4 (see JIRA). This replaces MutableBoolean with AtomicBoolean.

srowen pwendell

Author: Tathagata Das <tathagata.das1565@gmail.com>

Closes #5773 from tdas/SPARK-6752 and squashes the following commits:

a0c0ead [Tathagata Das] Fix for hadoop 1.0.4
70ae85b [Tathagata Das] Merge remote-tracking branch 'apache-github/master' into SPARK-6752
94db63c [Tathagata Das] Fix long line.
524f519 [Tathagata Das] Many changes based on PR comments.
eabd092 [Tathagata Das] Added Function0, Java API and unit tests for StreamingContext.getOrCreate
36a7823 [Tathagata Das] Minor changes.
204814e [Tathagata Das] Added StreamingContext.getOrCreate with existing SparkContext