Upgrade fee structure

[Reckless-Satoshi]

This PR upgrades the fee structure. So far it was very basic: the seller locks a hold invoice for X, the buyer submit an invoice of 0.998*X. The overall fee for a trade is 0.2%.

This is functional, however the fee load is fully applied to the buyer (...well the market can accommodate it by shifting the equilibrium premium). However, we would prefer to use a proper fee structure to incentivize liquidity providers to use the platform.

So the fee structure changes from:

	Maker	Taker
Buyer	0.2%	0.2%
Seller	0.0%	0.0%

To:

	Maker	Taker
Buyer	0.025%	0.175%
Seller	0.025%	0.175%

The total trade fee remains the same (0.2%), but now we align the maker/taker incentives to better fit the platform's need of makers.

Why exactly that split between maker/taker? This split is taken from the predominant BTC p2p market (Bisq): https://bisq.wiki/Trading_fees . The fees on Bisq are: 0.1% maker fee and 0.7% taker fee. There seems to have been a lot of deliberation around this split for Bisq (makers pay a fraction of 0.125 of the total fees), so it makes for a very a good initial value for RoboSats.

It all boils down to a new hparam: MAKER_FEE_SPLIT=0.125 (It is implicit that TAKER_FEE_SPLIT = 1 - MAKER_FEE_SPLIT)

[Reckless-Satoshi]

Everything works as expected. Already deployed in RoboSats mainnet.

For the future, one can think of also biasing the cost in the buyer-seller axis. As of now, buyer's routing fee is covered by the platform, while the seller's fee is not. In addition, there is an obvious imbalance between buyers and sellers.