db,view: guard view builder startup with a semaphore
db,view: guard view builder startup with a semaphore

The startup routine performs some bookkeeping operations on views,
and so do these events:
 - on_create_view;
 - on_drop_view;
 - on_update_view.
Since the above events are guarded with a semaphore, the startup
routine should also take the same semaphore - in order to ensure
that all bookkeeping operations are serialized.

Refs #6094

avikivity/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla,avikivity/scylla