Support paying fees in other assets

[stevenroose]

Currently fees are expected in the asset set in policyAsset. This is a single asset. It would be cool if this could be extended to a map of multiple assets and a multiplier back to the default one. Fees have no effect on consensus, so this is just a policy/mempool change. Fee estimation would be a bit more complicated, but I think even by only updating the multipliers sporadically, the fee estimation algo might actually allow for the funcs to slowly change the multiplier if the price changes.