Warn on unknown rw_settings

Summary:
Log a warning to debug log if unknown settings are encountered. This should probably only ever happen when the software is upgraded.

Something similar is already done for the command line and config file. See core#16234

This is a backport of core#19624

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D10541