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/protocol/encryption.go

@@ -153,7 +153,9 @@ func (e encryptedModel) DownloadProgress(p *DownloadProgress) error {
 		return e.model.DownloadProgress(p)
 	}
 
-	// Encrypted devices shouldn't send these - ignore them.
+	// We currently ignore these, though we could in principle translate
+	// them and use partially downloaded encrypted files like we do normal
+	// files.
 	return nil
 }