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 github.com:apache/spark into fix_conf