For view builder draining there's dedicated deferred action in main while all other services that need to be drained do it via storage_service. The latter is to unify shutdown for services and to make `nodetool drain` drain everything, not just some part of those. This PR makes view builder drain look the same. As a side effect it also moves `mark_existing_views_as_built` from storage service to view builder and generalizes this marking code inside view builder itself.
refs: #2737
refs: #2795Closesscylladb/scylladb#16558
* github.com:scylladb/scylladb:
storage_service: Drain view builder on drain too
view_builder: Generalize mark_as_built(view_ptr) method
view_builder: Move mark_existing_views_as_built from storage service
storage_service: Add view_builder& reference
main,cql_test_env: Move view_builder start up (and make unconditional)
298a7fcbf2