SQL: Add f.ttl column to control feed cache duration

Preparation of https://github.com/marienfressinaud/FreshRSS/issues/250
Will also be used to disable automatic update for selected feeds

app/SQL/install.sql.mysql.php

@@ -21,6 +21,7 @@ CREATE TABLE IF NOT EXISTS `%1$sfeed` (
 	`httpAuth` varchar(511) DEFAULT NULL,
 	`error` boolean DEFAULT 0,
 	`keep_history` MEDIUMINT NOT NULL DEFAULT -2,	-- v0.7
+	`ttl` INT NOT NULL DEFAULT -2,	-- v0.7.3
 	`cache_nbEntries` int DEFAULT 0,	-- v0.7
 	`cache_nbUnreads` int DEFAULT 0,	-- v0.7
 	PRIMARY KEY (`id`),

app/SQL/install.sql.sqlite.php

@@ -19,6 +19,7 @@ $SQL_CREATE_TABLES = array(
 	`httpAuth` varchar(511) DEFAULT NULL,
 	`error` boolean DEFAULT 0,
 	`keep_history` MEDIUMINT NOT NULL DEFAULT -2,
+	`ttl` INT NOT NULL DEFAULT -2,
 	`cache_nbEntries` int DEFAULT 0,
 	`cache_nbUnreads` int DEFAULT 0,
 	FOREIGN KEY (`%1$scategory`) REFERENCES `%1$scategory`(`id`) ON DELETE SET NULL ON UPDATE CASCADE,