The root cause for the hanging test is a concurrency deadlock.
`vector_store_client` runs dns refresh time and it is waiting for the condition
variable.After aborting dns request the test signals the condition variable.
Stopping the vector_store_client takes time enough to trigger the next dns
refresh - and this time the condition variable won't be signalled - so
vector_store_client will wait forever for finish dns refresh fiber.
The commit fixes the problem by waiting for the condition variable only once.
Fixes: #27237
Fixes: VECTOR-370
Closesscylladb/scylladb#27239
b5c85d08bb