Indexer hangs intermittently

This seems related to interactions between receiver cleanup and indexer processing.

2022-01-16T09:02:30.407-07:00	INFO thread=1 [receiver_indexer] requeueing notification index 'receiver_index'
2022-01-16T09:02:34.656-07:00	FINE thread=11 [receiver_indexer] running receiver cleanup
2022-01-16T09:17:30.498-07:00	Sun Jan 16 16:17:30 UTC 2022
2022-01-16T09:17:34.652-07:00	FINE thread=11 [receiver_indexer] running receiver cleanup
2022-01-16T09:32:30.561-07:00	Sun Jan 16 16:32:30 UTC 2022
2022-01-16T09:32:34.652-07:00	FINE thread=11 [receiver_indexer] running receiver cleanup
2022-01-16T09:47:30.616-07:00	Sun Jan 16 16:47:30 UTC 2022
2022-01-16T09:47:34.652-07:00	FINE thread=11 [receiver_indexer] running receiver cleanup
2022-01-16T10:02:30.421-07:00	Sun Jan 16 17:02:30 UTC 2022
2022-01-16T10:02:34.652-07:00	FINE thread=11 [receiver_indexer] running receiver cleanup
2022-01-16T10:17:30.480-07:00	Sun Jan 16 17:17:30 UTC 2022
2022-01-16T10:17:34.652-07:00	FINE thread=11 [receiver_indexer] running receiver cleanup
2022-01-16T10:32:30.578-07:00	Sun Jan 16 17:32:30 UTC 2022
2022-01-16T10:32:34.652-07:00	FINE thread=11 [receiver_indexer] running receiver cleanup