rpc: Make ValueFromAmount always return 8 decimals

This is the format that was always returned to JSON clients.
The difference was not noticed before, because VREAL values
are post-processed by univalue.

By implementing the functionality directly it breaks the dependency
of rpcserver on utilmoneystr. FormatMoney is now only used for debugging
purposes.

To test, port over the formatting tests from util_tests.cpp to
rpc_tests.cpp.