vector_store_client::stop did not properly clean up the
coroutine that was waiting for a notification on the refresh_client_cv
condition variable. As a result, the coroutine could try to access
`this` (via current_client) after the vector_store_client was destroyed.
To fix this, the `client_producer` tasks are wrapped by a gateway.
The `stop` method now signals the `client_producer` condition variable
and closes the gateway, which ensures that all `client_producer` tasks
are finished before the `stop` function returns.
The `wait_for_signal` return type was changed from `bool` to `void` as the return value was not used.
Fixes: VECTOR-230
Closesscylladb/scylladb#26076
bc06f89a5c