chore(protocol): minor cleanup of ClusterConfig messages; remove DisableTempIndexes option (#10202)

This makes a couple of backwards compatible changes to the
ClusterConfig:

- Remove the `ignore_permissions` and `ignore_delete` booleans which
we've never read or used for anything
- Remove the `disable_temp_indexes` boolean and option entirely. We did
use this one, and about 1% of users have set the option. The only thing
it does is inhibits sending of periodical DownloadProgress messages
while downloading data, which is a minuscule bandwidth optimisation
given that we're already sending data at the time.
- Change the `read_only` boolean (which indicated send-only folders) to
an enum `FolderType`, where the values zero and one match the existing
usage. Again, we don't actually use this value, but I can see that we
might want to and then it makes more sense for it to be more
comprehensive.
- Change the `paused` boolean to an enum `StopReason`, where zero
indicates not stopped and one indicates paused, exactly the same wire
representation as previously but leaves space for additional stop
reasons (errors etc).

lib/config/foldertype.go

@@ -6,13 +6,15 @@
 
 package config
 
-type FolderType int32
+import "github.com/syncthing/syncthing/lib/protocol"
+
+type FolderType protocol.FolderType
 
 const (
-	FolderTypeSendReceive      FolderType = 0
-	FolderTypeSendOnly         FolderType = 1
-	FolderTypeReceiveOnly      FolderType = 2
-	FolderTypeReceiveEncrypted FolderType = 3
+	FolderTypeSendReceive      = FolderType(protocol.FolderTypeSendReceive)
+	FolderTypeSendOnly         = FolderType(protocol.FolderTypeSendOnly)
+	FolderTypeReceiveOnly      = FolderType(protocol.FolderTypeReceiveOnly)
+	FolderTypeReceiveEncrypted = FolderType(protocol.FolderTypeReceiveEncrypted)
 )
 
 func (t FolderType) String() string {