[SPARK-4088] [PySpark] Python worker should exit after socket is closed by JVM

In case of take() or exception in Python, python worker may exit before JVM read() all the response, then the write thread may raise "Connection reset" exception.

Python should always wait JVM to close the socket first.

cc JoshRosen This is a warm fix, or the tests will be flaky, sorry for that.

Author: Davies Liu <davies@databricks.com>

Closes #2941 from davies/fix_exit and squashes the following commits:

9d4d21e [Davies Liu] fix race
Revert "[SPARK-4056] Upgrade snappy-java to 1.1.1.5"

This reverts commit 898b22ab1fe90e8a3935b19566465046f2256fa6.

Reverting because this may be causing OOMs.
[SPARK-4071] Unroll fails silently if BlockManager is small

In tests, we may want to have BlockManagers of size < 1MB (spark.storage.unrollMemoryThreshold). However, these BlockManagers are useless because we can't unroll anything in them ever. At the very least we need to log a warning.

tdas

Author: Andrew Or <andrew@databricks.com>

Closes #2917 from andrewor14/unroll-safely-logging and squashes the following commits:

38947e3 [Andrew Or] Warn against starting a block manager that's too small
fd621b4 [Andrew Or] Warn against failure to reserve initial memory threshold
SPARK-3359 [DOCS] sbt/sbt unidoc doesn't work with Java 8

This follows https://github.com/apache/spark/pull/2893 , but does not completely fix SPARK-3359 either. This fixes minor scaladoc/javadoc issues that Javadoc 8 will treat as errors.

Author: Sean Owen <sowen@cloudera.com>

Closes #2909 from srowen/SPARK-3359 and squashes the following commits:

f62c347 [Sean Owen] Fix some javadoc issues that javadoc 8 considers errors. This is not all of the errors turned up when javadoc 8 runs on output of genjavadoc.
Merge branch 'master' of git://git.apache.org/spark into spark-sql-dsl-improvement2