lib/model: Add global request limiter (fixes #6302) (#6303)

This adds a new config with the simple and concise name
maxConcurrentIncomingRequestKiB. This limits how many bytes we have "in
the air" in the form of response data being read and processed.

After some testing I think that not having this limiter is seldom a
great idea and thus I propose a default value of 256 MiB for this new
setting.

I also refactored the folder IO limiter to be a model/folder attribute
instead of a package global.

lib/model/folder.go

@@ -33,15 +33,12 @@ import (
 	"github.com/thejerf/suture"
 )
 
-// folderIOLimiter limits the number of concurrent I/O heavy operations,
-// such as scans and pulls. A limit of zero means no limit.
-var folderIOLimiter = newByteSemaphore(0)
-
 type folder struct {
 	suture.Service
 	stateTracker
 	config.FolderConfiguration
 	*stats.FolderStatisticsReference
+	ioLimiter *byteSemaphore
 
 	localFlags uint32
 
@@ -79,11 +76,12 @@ type puller interface {
 	pull() bool // true when successfull and should not be retried
 }
 
-func newFolder(model *model, fset *db.FileSet, ignores *ignore.Matcher, cfg config.FolderConfiguration, evLogger events.Logger) folder {
+func newFolder(model *model, fset *db.FileSet, ignores *ignore.Matcher, cfg config.FolderConfiguration, evLogger events.Logger, ioLimiter *byteSemaphore) folder {
 	return folder{
 		stateTracker:              newStateTracker(cfg.ID, evLogger),
 		FolderConfiguration:       cfg,
 		FolderStatisticsReference: stats.NewFolderStatisticsReference(model.db, cfg.ID),
+		ioLimiter:                 ioLimiter,
 
 		model:   model,
 		shortID: model.shortID,
@@ -303,8 +301,8 @@ func (f *folder) pull() bool {
 	f.setState(FolderSyncWaiting)
 	defer f.setState(FolderIdle)
 
-	folderIOLimiter.take(1)
-	defer folderIOLimiter.give(1)
+	f.ioLimiter.take(1)
+	defer f.ioLimiter.give(1)
 
 	return f.puller.pull()
 }
@@ -342,8 +340,8 @@ func (f *folder) scanSubdirs(subDirs []string) error {
 	f.setError(nil)
 	f.setState(FolderScanWaiting)
 
-	folderIOLimiter.take(1)
-	defer folderIOLimiter.give(1)
+	f.ioLimiter.take(1)
+	defer f.ioLimiter.give(1)
 
 	for i := range subDirs {
 		sub := osutil.NativeFilename(subDirs[i])