Snapshot/Restore: restore with wait_for_completion=true should wait for succesfully restored shards to get started

This commit ensures that restore operation with wait_for_completion=true doesn't return until all successfully restored shards are started. Before it was returning as soon as restore operation was over, which cause some shards to be unavailable immediately after restore completion.

Fixes #8340
hmm can we add a `break` here somehow? I think we are looking for exactly one? Oh I see it's collecting the others below so I think it would be good to add an assertion that the one we are looking for is in here only once?