The `raft_address_map` code was "clever": it used two intrusive data structures and did a lot of manual lifetime management; raw pointer manipulation, manual deletion of objects... It wasn't clear who owns which object, who is responsible for deleting what. And there was a lot of code.
In this PR we replace one of the intrusive data structures with a good old `std::unordered_map` and make ownership clear by replacing the raw pointers with `std::unique_ptr`. Furthermore, some invariants which were not clear and enforced in runtime are now encoded in the type system.
The code also became shorter: we reduced its length from ~360 LOC to ~260 LOC.
Closes#11763
* github.com:scylladb/scylladb:
service/raft: raft_address_map: get rid of `is_linked` checks
service/raft: raft_address_map: get rid of `to_list_iterator`
service/raft: raft_address_map: simplify ownership of `expiring_entry_ptr`
service/raft: raft_address_map: move _last_accessed field from timestamped_entry to expiring_entry_ptr
service/raft: raft_address_map: don't use intrusive set for timestamped entries
service/raft: raft_address_map: store reference to `timestamped_entry` in `expiring_entry_ptr`
ee2398960c