Convert schedule functions for IAmBored to single execution

[LexLuthr]

We do scheduling for IAmBored in a schedule function. Typically, this looks like below.

addTaskFunc(func(taskID harmonytask.TaskID, tx *harmonydb.Tx) (shouldCommit bool, seriousError error) {
			stop = true

			var pendings []struct {
				ID int64 `db:"id"`
			}

			err := tx.Select(&pendings, `SELECT id
				FROM pdp_delete_data_set
				WHERE cleanup_pieces_task_id IS NULL
				  AND cleanup_pieces_tx_hash IS NULL
				  AND after_delete_data_set = TRUE
				  AND delete_tx_hash IS NULL
				  AND service_termination_epoch IS NOT NULL
				  AND terminated = FALSE
				ORDER BY id`)
			if err != nil {
				return false, xerrors.Errorf("failed to select pending PDP cleanup data sets: %w", err)
			}

			if len(pendings) == 0 {
				log.Debugw("no pending PDP data sets for piece cleanup")
				return false, nil
			}

			pending := pendings[0]

			n, err := tx.Exec(`UPDATE pdp_delete_data_set
				SET cleanup_pieces_task_id = $1
				WHERE id = $2
				  AND cleanup_pieces_task_id IS NULL
				  AND cleanup_pieces_tx_hash IS NULL
				  AND after_delete_data_set = TRUE
				  AND delete_tx_hash IS NULL
				  AND service_termination_epoch IS NOT NULL
				  AND terminated = FALSE`, taskID, pending.ID)
			if err != nil {
				return false, xerrors.Errorf("failed to assign PDP cleanup task: %w", err)
			}
			if n != 1 {
				return false, xerrors.Errorf("updated %d rows assigning PDP cleanup task", n)
			}

			log.Debugw("scheduled PDP cleanupPieces task", "dataSetId", pending.ID)
			stop = false
			return true, nil
		})

We can combine above in a single SQL execution with just setting the ID for first row with condition and then return true if n == 1. This pattern can be used for all schedule function which do not require additional processing before assigning the task.