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/config/migrations.go

@@ -25,6 +25,7 @@ import (
 // update the config version. The order of migrations doesn't matter here,
 // put the newest on top for readability.
 var migrations = migrationSet{
+	{30, migrateToConfigV30},
 	{29, migrateToConfigV29},
 	{28, migrateToConfigV28},
 	{27, migrateToConfigV27},
@@ -84,6 +85,13 @@ func (m migration) apply(cfg *Configuration) {
 	cfg.Version = m.targetVersion
 }
 
+func migrateToConfigV30(cfg *Configuration) {
+	// The "max concurrent scans" option is now spelled "max folder concurrency"
+	// to be more general.
+	cfg.Options.RawMaxFolderConcurrency = cfg.Options.DeprecatedMaxConcurrentScans
+	cfg.Options.DeprecatedMaxConcurrentScans = 0
+}
+
 func migrateToConfigV29(cfg *Configuration) {
 	// The new crash reporting option should follow the state of global
 	// discovery / usage reporting, and we should display an appropriate