[SPARK-4079] [CORE] Default to LZF if Snappy not available

By default, snappy is the compression codec used. If Snappy is not available, Spark currently throws a stack trace. Now Spark falls back to LZF if Snappy is not available on the cluster and logs a warning message.

The only exception is if the user has explicitly set spark.io.compression.codec=snappy. In this case, if snappy is not available, an IllegalArgumentException is thrown.

Because of the way the Snappy library uses static initialization, it was very difficult in a unit test to simulate Snappy not being available. The only way I could think of was to create multiple classloaders which seemed excessive. As a result, most of this was tested adhoc on a test cluster by modifying the system property: org.xerial.snappy.use.systemlib=true which caused Snappy to not load and thus triggering this logic.
nit: indented too far (Spark's convention is 2 spaces for the continuation line).