Fix possible memory corruption in FLUSHALL when a client watches more than one key (#11854)

Avoid calling unwatchAllKeys when running touchAllWatchedKeysInDb (which was unnecessary)
This can potentially lead to use-after-free and memory corruption when the next entry
pointer held by the watched keys iterator is freed when unwatching all keys of a specific client.
found with address sanitizer, added a test which will not always fail (depending on the random
dict hashing seed)
problem introduced in #9829 (Reids 7.0)

Co-authored-by: Oran Agra <oran@redislabs.com>