Stable price support2

[ianhe8x]

No description provided.

[ianhe8x]

@Jianzhuo We need to enhance security of oracle contract

• Add controller role, owner can change controller. (because we might need to put the pk in a unsecured env so it runs automatically.
• When controller set a price, we need to limit the change rate as well as interval
  • changes (+-)x% in y blocks, only owner can set these two parameters.
  • x default to 10, y default to 3600 (2 hrs)
• Owner have a different method to set price, and without limitation