This series is a step towards non-LRU cache algorithms.
Our cache items are able to unlink themselves from the LRU list. (In other words, they can be unlinked solely via a pointer to the item, without access to the containing list head). Some places in the code make use of that, e.g. by relying on auto-unlink of items in their destructor.
However, to implement algorithms smarter than LRU, we might want to update some cache-wide metadata on item removal. But any cache-wide structures are unreachable through an item pointer, since items only have access to themselves and their immediate neighbours. Therefore, we don't want items to unlink themselves — we want `cache.remove(item)`, rather than `item.remove_self()`, because the former can update the metadata in `cache`.
This series inserts explicit item unlink calls in places that were previously relying on destructors, gets rid of other self-unlinks, and adds an assert which ensures that every item is explicitly unlinked before destruction.
Closes#11716
* github.com:scylladb/scylladb:
utils: lru: assert that evictables are unlinked before destruction
utils: lru: remove unlink_from_lru()
cache: make all cache unlinks explicit
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