Add option `-rpcamount` to set format for monetary amounts in RPC

Support this option at both the client side and server side. Both sides need
to be using the same value of this option, otherwise parse errors
will ensue.

Four modes are available:

- number-btc: Format as number in decimal BTC (default)
- number-satoshis: Format as number in satoshis
- string-btc: Format as string in decimal BTC
- string-satoshis: Format as string in satoshis

Also add tests for AmountFromValue / ValueFromAmount for all four modes.