Incorporate pricing into O selection

Currently, we filter out orchestrators higher than the broadcaster's maximum price. However, there may still be orchestrators within that set which offer a significant cost savings relative to others.

We should offer options to take pricing and other economic factors into consideration when selecting Os, on top of latency. These factors may involve: the price per pixel, the ticket EV, the face value. The approach for doing this should be determined before starting any implementation.

Related: Latency based O selection https://github.com/livepeer/go-livepeer/issues/938

Start by reviewing the related latency-based O selection issue, #938, and the current orchestrator selection behavior described here. First define how price per pixel, ticket EV, face value, and latency should be weighed; done means an agreed approach is documented before implementation begins.

Written by the indexing model from the issue text.