The caller is table with view-update-generator at hand (it calls
mutate_MV on). Builder here is used as a temporary object that destroys
once the caller coroutine co_return-s, so keeping the database obtained
from the view-update-generator is safe.
Later the v.u.b. object will propagate its data dictionary down the
callstacks.
Signed-off-by: Pavel Emelyanov <xemul@scylladb.com>