Persistent settings are used in followup PRs #15936 to unify gui settings
between bitcoin-qt and bitcoind, and #15937 to add a load_on_startup flag to
the loadwallet RPC and maintain a dynamic list of wallets that should be loaded
on startup that also can be shared between bitcoind and bitcoin-qt.
Implement merging of settings from different sources (command line and config
file) separately from parsing code in system.cpp, so it is easier to add new
sources.
Document current inconsistent merging behavior without changing it.
This commit only adds new settings code without using it. The next commit calls
the new code to replace existing code in system.cpp.
Co-authored-by: John Newbery <john@johnnewbery.com>
