Generate canary events twice an hour
The new Refine process senses _IMPORTED flags generated by Gobblin.
But some streams imported with Gobblin got missing _IMPORTED flags for some partitions. When there are no events in the previous hour, the Gobblin code does not add the flag for the last hour. When there is a single canary event per hour, this could happen.
Let's produce canary events twice an hour to prevent this problem.
Bug: T365223