Partial backport (9/11) of core [[https://github.com/bitcoin/bitcoin/pull/18918 | PR18918]]:
https://github.com/bitcoin/bitcoin/pull/18918/commits/9ea2d258b46e8a9776100633585ed0feede5c2a4
Depends on D8595.
The DBKeys declaration are added to the `walletdb.h` file to make them
available for `salvage.cpp`, see D7020.