New BSIP: Prediction Market Improvements

Related discussions:
* https://github.com/bitshares/bitshares-core/issues/460
* https://github.com/bitshares/bitshares-core/issues/1776

Current behavior:
* When the outcome of a PM is 1, the issuer can globally settle with 1, the price feed producers can feed 1 as well (which will trigger a black swan event then globally settle at 1), the result is same.
* When the outcome of a PM is 0, the issuer can globally settle with 0, but
* the price feed producers are unable to feed 0 (which is a bit complicated to fix),
* even if they feed a very small price e.g. 0.0001, the market won't be globally settled since a small price won't trigger a black swan event.

The final fix would be:
* allow and only allow feeding 0 or 1
* when there are enough feeds, global settle at median feed price.

By the way, settling at 0 means the market fees collected by the asset issuer/creator and shared to referral program worth zero. That's not a good business.

Core exchange rate of PM need to be considered as well. I think the best approach is to claim all CORE asset from the fee pool thus prevent the asset from being used to pay fees.

Start by reading the two related BitShares Core discussions and the current-behavior examples in this proposal. Define the protocol and fee-pool behavior that must change, including valid feeds, median-feed settlement, and CORE handling; done means the proposal resolves these cases consistently.

Written by the indexing model from the issue text.