refactor: optimize modifiers

[pegahcarter]

For each function a modifier is attached to, the bytecode of the full modifier is added (reference here). So, by adding a private/internal function call, the bytecode of the logic statements is only copied to the contract once and each modifier only references the function call.

I went ahead and turned the modifiers we've made into private func calls. I left onlyOwner as-is to maintain the Synthetix code. One thing I'm wondering- what are your thoughts of removing Owned.sol in favor of OZ's Ownable2Step?