lib/config, lib/model: Limit concurrent pulls (fixes #5914) (#6290)

Adds a new folder state "Waiting to Sync" in the same vein as the
existing "Waiting to Scan". This vastly improves performances in the
rare cases when there are lots and lots of folders operating.

lib/model/folderstate.go

@@ -19,6 +19,7 @@ const (
 	FolderIdle folderState = iota
 	FolderScanning
 	FolderScanWaiting
+	FolderSyncWaiting
 	FolderSyncPreparing
 	FolderSyncing
 	FolderError
@@ -32,6 +33,8 @@ func (s folderState) String() string {
 		return "scanning"
 	case FolderScanWaiting:
 		return "scan-waiting"
+	case FolderSyncWaiting:
+		return "sync-waiting"
 	case FolderSyncPreparing:
 		return "sync-preparing"
 	case FolderSyncing: