Action Pack: Possible restoration of destroyed sessions in Action Dispatch CacheStore

### Impact `ActionDispatch::Session::CacheStore` writes a session back to the cache at the end of each request that loaded it, without checking that the session's cache entry still exists. When a concurrent request destroys the session, through `reset_session` on logout or by deleting its cache entry on the server, a request that loaded the session earlier and finishes later restores it under its original session ID. Logout and server-side revocation could therefore fail to end a session, and an attacker who holds a stolen session cookie could keep that session valid across the victim's logout. Applications that use `config.session_store :cache_store` are affected, with any cache backend. `ActionDispatch::Session::CookieStore` is not affected, nor is `ActionDispatch::Session::MemCacheStore` with Dalli 4.0.0 or later, whose session store does not write back a session that was deleted. ### Details The fixed releases record which session a request loaded and skip the write when that session's cache entry no longer exists. Changes that such a request makes to the destroyed session are discarded, and Rack logs a warning that the session could not be saved. Each request that writes back a session it loaded makes one additional cache read. ### Releases The fixed releases are available at the normal locations. Patches for those who cannot upgrade at once: - 8.1: https://github.com/rails/rails/commit/d479192cf88a8b30a335a8bc25188a2d8e0f8d9f.patch - 8.0: https://github.com/rails/rails/commit/5bb5ec66ed8faaf590ffe1222d7a5eb6e9d050f5.patch ### Credit Found by the team during internal code review.