We move a `seastar::promise` on the external worker thread,
after the matching `seastar::future` was returned to the shard.
That's illegal. If the `promise` move occurs concurrently with some
operation (move, await) on the `future`, it becomes a data race
which could cause various kinds of corruption.
This patch fixes that by keeping the promise at a stable address
on the shard (inside a coroutine frame) and only passing through
the worker.
Fixes#24751Closesscylladb/scylladb#24752
(cherry picked from commit a29724479a)
Closesscylladb/scylladb#24777
Introduces a util which launches a new OS thread and accepts
callables for concurrent execution.
Meant to be created once at startup and used until shutdown,
for running nonpreemptible, 3rd party, non-interactive code.
Note: this new utility is almost identical to wasm::alien_thread_runner.
Maybe we should unify them.
