lib/model: Mark initial scan as finished even if failed and refactor (fixes #4103)

The mechanism to disallow manual scans before the initial scan completed
(#3996) , had the side effect, that if the initial scan failed, no further
scans are allowed. So this marks the initial scan as finished regardless of
whether it succeeded or not.

There was also redundant code in rofolder and a pointless check for folder
health in scanSubsIfHealthy (happens in internalScanFolderSubdirs as well).
This also moves logging from folder.go to ro/rw-folder.go to include the
information about whether it is send-only or send-receive

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/4104

lib/model/rwfolder_test.go

@@ -80,9 +80,9 @@ func setUpModel(file protocol.FileInfo) *Model {
 func setUpSendReceiveFolder(model *Model) *sendReceiveFolder {
 	f := &sendReceiveFolder{
 		folder: folder{
-			stateTracker:         newStateTracker("default"),
-			model:                model,
-			initialScanCompleted: make(chan struct{}),
+			stateTracker:        newStateTracker("default"),
+			model:               model,
+			initialScanFinished: make(chan struct{}),
 		},
 
 		mtimeFS:   fs.NewMtimeFS(fs.DefaultFilesystem, db.NewNamespacedKV(model.db, "mtime")),
@@ -93,7 +93,7 @@ func setUpSendReceiveFolder(model *Model) *sendReceiveFolder {
 	}
 
 	// Folders are never actually started, so no initial scan will be done
-	close(f.initialScanCompleted)
+	close(f.initialScanFinished)
 
 	return f
 }