accounting: Add desired units to node report query

[carlaKC]

At present, the node report output query provides BTC values quoted in millisatoshis.
Multiple amount fields can be confusing, so rather than add more fields with our new units, we can deprecate the existing field in ReportEntry and replace it with a oneof. Since fiat values are subject to rounding (if we round down to BTC, we likely also want to round down our USD amount), we should paid the amount + fiat fields. Calculation in the actual accounting package should remain in msat, since it provides us with the best level of precision.

Steps to completion

1. Add a unit enum to NodeReportRequest which defaults to msat but allows satoshis and bitcoin, round accordingly in rpcReportResponse and make sure the headings in the csv file are updated
2. Deprecate the amount and fiat fields and replace with a oneof:

entryAmount{
  amount = 1;
  fiat = 2;
}

oneOf{
  entryAmount msatAmount =1;
  entryAmount satoshiAmount = 2; 
  entryAmount bitcoinAmount =3; 
}

This change should be mindful of the intention to add other fiat currencies, as detailed in #38