Update SqlPlanNode.copy() to copy ancestor nodes

[plypaul]

• For the nodes in a SqlPlan, the invariant is that there are no duplicate nodes.
• With the recent caching updates that allow for more reuse, copying all nodes in the hierarchy is simpler to reason for correctness.
• With the update to do additional copying, the node IDs in the snapshots have changed.
• The additional copy overhead should be small as there aren't that many node in a SQL plan for a query (<100), though the overhead can grow ~n^2 with the number of nodes.
• New copies of the fields in the nodes are not created. Since they are much more numerous, creating copies of those could have a more significant impact.