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