This series greatly reduces gossipers' dependence on `seastar::async` (yet, not completely).
`i_endpoint_state_change_subscriber` callbacks are converted to return futures (again, to get rid of `seastar::async` dependency), all users are adjusted appropriately (e.g. `storage_service`, `cdc::generation_service`, `streaming::stream_manager`, `view_update_backlog_broker` and `migration_manager`).
This includes futurizing and coroutinizing the whole function call chain up to the `i_endpoint_state_change_subscriber` callback functions.
To aid the conversion process, a non-`seastar::async` dependent variant of `utils::atomic_vector::for_each` is introduced (`for_each_futurized`). A different name is used to clearly distinguish converted and non-converted code, so that the last step (remove `seastar::async()` wrappers around callback-calling code in gossiper) is easier. This is left for a follow-up series, though.
Tests: unit(dev)
Closes#9844
* github.com:scylladb/scylla:
service: storage_service: coroutinize `set_gossip_tokens`
service: storage_service: coroutinize `leave_ring`
service: storage_service: coroutinize `handle_state_left`
service: storage_service: coroutinize `handle_state_leaving`
service: storage_service: coroutinize `handle_state_removing`
service: storage_service: coroutinize `do_drain`
service: storage_service: coroutinize `shutdown_protocol_servers`
service: storage_service: coroutinize `excise`
service: storage_service: coroutinize `remove_endpoint`
service: storage_service: coroutinize `handle_state_replacing`
service: storage_service: coroutinize `handle_state_normal`
service: storage_service: coroutinize `update_peer_info`
service: storage_service: coroutinize `do_update_system_peers_table`
service: storage_service: coroutinize `update_table`
service: storage_service: coroutinize `handle_state_bootstrap`
service: storage_service: futurize `notify_*` functions
service: storage_service: coroutinize `handle_state_replacing_update_pending_ranges`
repair: row_level_repair_gossip_helper: coroutinize `remove_row_level_repair`
locator: reconnectable_snitch_helper: coroutinize `reconnect`
gms: i_endpoint_state_change_subscriber: make callbacks to return futures
utils: atomic_vector: introduce future-returning `for_each` function
utils: atomic_vector: rename `for_each` to `thread_for_each`
gms: gossiper: coroutinize `start_gossiping`
gms: gossiper: coroutinize `force_remove_endpoint`
gms: gossiper: coroutinize `do_status_check`
gms: gossiper: coroutinize `remove_endpoint`
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