[prediction-markets] Refund when destroying a market

[maltekliemann]

Based on a disccusion with @Robiquet.

When destroying a market, we should consider keeping the pool of the market alive, using the funds from the market account to refund everybody's outcome tokens at market value and letting people withdraw their funds.

Since the sum of outcome token spot prices might currently exceed 1, we cannot do this until we have a no-arbitrage AMM.