Prevent MEV with streaming opcodes

[PaulRBerg]

The gas prices for the streaming opcodes should not allow node operators to profit by 'sandwiching' users (e.g. by introducing another stream in front of the user's stream).

This is more like a hypothesis for an open research problem. I'm not convinced that this will actually be a problem. We will have more clarity on this once we implement the opcodes and tinker with their gas prices.

Context: this was pointed out by Pranav Garimidi when we met at CSX.