rschlussel and rschlussel2 Replace distributed_join with join_distribution_type session property
The `join_distribution_type` property has three option: "REPARTITIONED",
"BROADCAST", and "AUTOMATIC". This property replaces the
`distributed_join` property. "REPARTITIONED" has the same behavior as
`distribtued_join=true`. "BROADCAST" is equivalent to
`distributed_join=false`. "AUTOMATIC" will use stats to evaluate whether
partitioned or replicated is better, or if no information is available,
it will fall back to partitioned.

The default value for `join_distribution_type` is "REPARTITIONED".

For backwards compatibility, wwhen the old `distribtued_join` session
property is set, it supercedes the new `join_distribution_type`
property.  The default value for `distributed_join` is now null. We plan
to remove it in a future release.
Latest commit 506f457 Feb 8, 2018