Outbailment id sort

This PR adds the sorting of []*OutputReq by OutBailmentID. It also adds explicit modelling of the OutBailmentID as an 'sortByOutBailmentID' seemed confusing without any definition of what an OutBailmentID was.
Yes, looking I've found the same convention in btcwallet and even in the sort examples in the sort package documetation, so that's done. I'm not a fan of one-liners as I think they are harder to read.