fix issues uncovered as part of basic 3-way join test:
1. many places need access to a RelNode to be wraped with
getNonSubsetRelNode, so we get the best Node from a Subset.
2. CBO.apply: reinitialize Planner/Cluster to have the RelBucketing
trait. Add caching provider to cluster.
3. HiveIRRel classes: add copy method.
4. ConvertToCommonJoin: add RelBucketing trait to new RelNode.
5. SwapJoinRule: construct a HiveProjRel, not a ProjeRel
6. RelBucketingTraitImpl: fix equals method, set numBuckets,
szLargeBucket to 0 for EMPTY instance.