Block until queued rows have been written to BigQuery, or timeout.
Useful between request boundaries when callers want each invocation's
events to be durable before returning. Does NOT shut the handler
down; subsequent events keep working. Logs a warning and returns
without raising if timeout elapses with rows still in flight.
queue.Queue.join() does not accept a timeout, so we replicate its
condition-variable wait pattern explicitly using the queue's
all_tasks_done Condition (de facto stable across CPython
versions; the same attribute Queue.join itself uses).
