Upgrade Arrow to version 0.8.0 and upgrade Netty to 4.1.17

Arrow version 0.8.0 is slated for release in early November, but I'd like to start discussing to help get all the work that's being done synced up.

Along with upgrading the Arrow Java artifacts, pyarrow on our Jenkins test envs will need to be upgraded as well that will take a fair amount of work and planning.

One topic I'd like to discuss is if pyarrow should be an installation requirement for pyspark, i.e. when a user pip installs pyspark, it will also install pyarrow. If not, then is there a minimum version that needs to be supported? We currently have 0.4.1 installed on Jenkins.

There are a number of improvements and cleanups in the current code that can happen depending on what we decide (I'll link them all here later, but off the top of my head):

• Decimal bug fix and improved support
• Improved internal casting between pyarrow and pandas (can clean up some workarounds), this will also verify data bounds if the user specifies a type and data overflows. see https://github.com/apache/spark/pull/19459#discussion_r146421804
• Better type checking when converting Spark types to Arrow
• Timestamp conversion to microseconds (for Spark internal format)
• Full support for using validity mask with 'object' types https://github.com/apache/spark/pull/18664#discussion_r146567335
• VectorSchemaRoot can call close more than once to simplify listener https://github.com/apache/spark/blob/master/sql/core/src/main/scala/org/apache/spark/sql/execution/arrow/ArrowConverters.scala#L90