+ * When updates are deferred, they go into one two FIFO "top-queues" (one for pre-send and one
+ * for post-send). Updates enqueued *during* doUpdate() of a "top" update go into the "sub-queue"
+ * for that update. After that method finishes, the sub-queue is run until drained. This continues
+ * for each top-queue job until the entire top queue is drained. This happens for the pre-send
+ * top-queue, and later on, the post-send top-queue, in execute().