[SPARK-9142][SQL] remove more self type in catalyst

[cloud-fan]

a follow up of #7479.
The TreeNode is the root case of the requirement of self: Product => stuff, so why not make TreeNode extend Product?

[SparkQA]

Test build #37715 has finished for PR 7495 at commit 8676af7.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds the following public classes (experimental):
  • abstract class Expression extends TreeNode[Expression]
  • abstract class LogicalPlan extends QueryPlan[LogicalPlan] with Logging
  • abstract class TreeNode[BaseType <: TreeNode[BaseType]] extends Product
  • abstract class SparkPlan extends QueryPlan[SparkPlan] with Logging with Serializable

[rxin]

Thanks - merging this in.