Permanently mark daemons and clusters which use experimental features
We want to know if a cluster we are debugging has ever used experimental features. Probably we can dump them as part of "ceph -s" et al (not as a warning, just information dump).
Preferably, it lists both currently enabled features and features which have been enabled in the past, and daemons which have permanent state store their list of enabled-in-the-past features on disk and expose them via logs on startup, as well as any other appropriate channels.
For the MDS, this probably means it has to modify the mdsmap some way or other; the OSDs and monitors can obviously do this in their stores.