Make Plan's `OnNode` mutable

[takaoka-youta-opst]

Proposal

Make Plan's OnNode mutable.

OnNode claims node where Runs of the Plan should be run on.

"Which node has been used" is not related with lineage, so changing this does not effect lineage tracking or reproductibity.

[takaoka-youta-opst]

Now, OnNode is counted as Plan Hash source.
If OnNode is mutable, it should not effect to Hash.

If we do this, we need rehash Plans.