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/folder_sendrecv.go

@@ -140,25 +140,6 @@ func newSendReceiveFolder(model *model, fset *db.FileSet, ignores *ignore.Matche
 // pull returns true if it manages to get all needed items from peers, i.e. get
 // the device in sync with the global state.
 func (f *sendReceiveFolder) pull() bool {
-	select {
-	case <-f.initialScanFinished:
-	default:
-		// Once the initial scan finished, a pull will be scheduled
-		return true
-	}
-
-	// If there is nothing to do, don't even enter pulling state.
-	abort := true
-	snap := f.fset.Snapshot()
-	snap.WithNeed(protocol.LocalDeviceID, func(intf db.FileIntf) bool {
-		abort = false
-		return false
-	})
-	snap.Release()
-	if abort {
-		return true
-	}
-
 	if err := f.CheckHealth(); err != nil {
 		l.Debugln("Skipping pull of", f.Description(), "due to folder error:", err)
 		return false