In main.cc storage_service is started before and stopped after
repair_service. storage_service keeps a reference to sharded
repair_service and calls its methods, but nothing ensures that
repair_service's local instance would be alive for the whole
execution of the method.
Add a gate to repair_service and enter it in storage_service
before executing methods on local instances of repair_service.
Fixes: #21964.
Closesscylladb/scylladb#22145
32ab58cdea