Some time ago it turned out that if unrecognized feature name is met in scylla.yaml, the whole experimental features list is ignored, but scylla continues to boot. There's UNUSED feature which is the proper way to deprecate a feature, and this PR improves its handling in several ways.
1. The recently removed "tablets" feature is partially brought back, but marked as UNUSED
2. Any UNUSED features met while parsing are printed into logs
3. The enum_option<> helper is enlightened along the way
refs: #18968
(cherry picked from commit f56cdb1cac)
(cherry picked from commit 0c0a7d9b9a)
(cherry picked from commit b85a02a3fe)
(cherry picked from commit b2520b8185)
Refs #19230Closesscylladb/scylladb#19266
* github.com:scylladb/scylladb:
config: Mark tablets feature as unused
main: Warn unused features
enum_option: Carry optional key on board
enum_option: Remove on-board _map member
5139e74058