backups: remove entries from database that don't exist in storage

fixes #772

src/backupdb.js

@@ -18,6 +18,7 @@ exports = module.exports = {
     get,
     del,
     update,
+    list,
 
     _clear: clear
 };
@@ -80,6 +81,21 @@ function getByIdentifierPaged(identifier, page, perPage, callback) {
         });
 }
 
+function list(page, perPage, callback) {
+    assert(typeof page === 'number' && page > 0);
+    assert(typeof perPage === 'number' && perPage > 0);
+    assert.strictEqual(typeof callback, 'function');
+
+    database.query('SELECT ' + BACKUPS_FIELDS + ' FROM backups ORDER BY creationTime DESC LIMIT ?,?',
+        [ (page-1)*perPage, perPage ], function (error, results) {
+            if (error) return callback(new BoxError(BoxError.DATABASE_ERROR, error));
+
+            results.forEach(function (result) { postProcess(result); });
+
+            callback(null, results);
+        });
+}
+
 function get(id, callback) {
     assert.strictEqual(typeof id, 'string');
     assert.strictEqual(typeof callback, 'function');