Tradinglimits account for overall inbound/outbound balance

[kilrau]

As a follow-up of https://github.com/ExchangeUnion/xud/pull/1933 we want to remove the single max channel restriction and have all inbound/outbound channels accumulated be the max buy/sell.

Depends on https://github.com/ExchangeUnion/xud/issues/1590 & https://github.com/ExchangeUnion/xud/issues/1975