Improve empty-queue behaviour, reduce stampeding

src/database-backends/postgresql/index.js

@@ -23,7 +23,8 @@ module.exports = function(state) {
 	return {
 		defaultSettings: {
 			taskBatchSize: 1000,
-			taskBatchDelay: 30 * 1000
+			taskBatchDelay: 5 * 60 * 1000, // FIXME: Rename to reflect changed behaviour?
+			delayRandomization: 0.2
 		},
 		create: function createPostgreSQLBackend(options) {
 			let knex = knexLibrary({

src/database-backends/postgresql/queries/get-task-stream.js

@@ -143,7 +143,8 @@ module.exports = function ({ metrics, backendSettings, knex }) {
 						}).then((newItems) => {
 							if (newItems === 0) {
 								// TODO: Consider using LISTEN/NOTIFY instead? Worth the added complexity?
-								return Promise.resolve([]).delay(backendSettings.taskBatchDelay);
+								let randomization = Math.random() * backendSettings.delayRandomization * backendSettings.taskBatchDelay; // To prevent stampeding by low-throughput tasks
+								return Promise.resolve([]).delay(backendSettings.taskBatchDelay + randomization);
 							} else {
 								// Have another go right away
 								return [];