useNamespace method
Puts every queue this process dispatches to, works, lists or flushes
under namespace, as <namespace>.<queue>. Null for none.
In a preview, read from DARTVEL_QUEUE_NAMESPACE when nothing set it,
which is what a preview deployment writes, and ignored in every other
environment: a preview on production's broker would
otherwise reserve from production's default queue and run production's
jobs against a seeded database, and nothing would throw. Applied here
rather than in each adapter, so an adapter added later cannot forget it.
A process in a preview with no namespace uses no queue at all, and a process with no namespace cannot name a preview's queue.
Implementation
void useNamespace(String? namespace) {
if (namespace != null) _checkNamespace(namespace);
_namespace = namespace;
_namespaceSettled = true;
}