update library grouped by source (#511)

* Update mangas grouped by source

* Limit parallel update requests

server/src/test/resources/server-reference.conf

@@ -10,6 +10,9 @@ server.socksProxyPort = ""
 # downloader
 server.downloadAsCbz = false
 
+# updater
+server.maxParallelUpdateRequests = 10
+
 # misc
 server.debugLogsEnabled = true
 server.systemTrayEnabled = false