persistent-nodemap: add a revlog.storage.persistent-nodemap.slow-path option As discussed during the sprint, we want to prevent user to get an unexpected performance regression when accessing a repository using "persistent-nodemap" without the associated Rust extension. We start by adding a config declaration and some documentation. Since "allow" is the current behavior, we don't need to add any code. The option possible value will come later. Note that we already have a `storage.revlog.nodemap.mode` option, but that option is a bit different. It does some warning and checking at revlog instantiation time. While we want something done at requirements checking time. Since we plan for new names and new config value names, we introduce a new option and will drop the old one later. Differential Revision: https://phab.mercurial-scm.org/D9758