Need support for rewriting grin-server.toml file

[tromp]

The fixfees RFC lowered the accept_fee_base default from 1_000_000 to 500_000, but most nodes
have the 1_000_000 setting in the grin-server.toml configuration file, which would incorrectly override the new 500_000 default.

This resulted in the ugly fight-override fix at #3539

To get rid of that exception code, we would like to be able to rewrite the grin-server.toml file
in order to update obsolete defaults.

[trevyn]

For reference, I started working on this over at trevyn/grin/commits/trevyn-fix-accept-fee-base. I'm not entirely happy with the approach yet, but I did get it working. I've put this on the back burner and may pick it up again, but until then, I wanted to inform anyone who might find this information to be useful. :)