Change create_payouts + create_trade_req to be more currency specific

[ericecook]

Currently it just creates all possible payouts/trade requests. This is frequently undesireable, due to an issue with one currency. Additionally, down the road it'd be nice to do more frequent payouts for some currencies - or even allow users to manually make a payout request/threshold/etc

Short term solution:

Abstract functions to expect currency to be passed in. Have calling function loop as desired.

Long term solution:

Further abstract functions to expect both user & currency to be passed, This allows much more flexible creation options.