mm: arbitrage bots need a profit "plus" option

[buck54321]

Arbitrage bots do not consider fee losses. We need something analogous to the percentage-plus gap strategy for the arbitrage profit threshold, where anticipated fee losses are added.

[martonp]

This is handled in https://github.com/decred/dcrdex/pull/2678. Although now they consider fees by default. We could still add an option to not consider fees.