lib/api: Improve folder summary event, verbose service (#9370)

This makes a couple of small improvements to the folder summary
mechanism:

- The folder summary includes the local and remote sequence numbers in
clear text, rather than some odd sum that I'm not sure what it was
intended to represent.
- The folder summary event is generated when appropriate, regardless of
whether there is an event listener. We did this before because
generating it was expensive, and we wanted to avoid doing it
unnecessarily. Nowadays, however, it's mostly just reading out
pre-calculated metadata, and anyway, it's nice if it shows up reliably
when running with -verbose.

The point of all this is to make it easier to use these events to judge
when devices are, in fact, in sync. As-is, if I'm looking at two
devices, it's very difficult to reliably determine if they are in sync
or not. The reason is that while we can ask device A if it thinks it's
in sync, we can't see if the answer is "yes" because it has processed
all changes from B, or if it just doesn't know about the changes from B
yet. With proper sequence numbers in the event we can compare the two
and determine the truth. This makes testing a lot easier.

lib/db/set.go

@@ -338,17 +338,22 @@ func (s *Snapshot) Sequence(device protocol.DeviceID) int64 {
 	return s.meta.Counts(device, 0).Sequence
 }
 
-// RemoteSequence returns the change version for the given folder, as
-// sent by remote peers. This is guaranteed to increment if the contents of
-// the remote or global folder has changed.
-func (s *Snapshot) RemoteSequence() int64 {
-	var ver int64
-
+// RemoteSequences returns a map of the sequence numbers seen for each
+// remote device sharing this folder.
+func (s *Snapshot) RemoteSequences() map[protocol.DeviceID]int64 {
+	res := make(map[protocol.DeviceID]int64)
 	for _, device := range s.meta.devices() {
-		ver += s.Sequence(device)
+		switch device {
+		case protocol.EmptyDeviceID, protocol.LocalDeviceID, protocol.GlobalDeviceID:
+			continue
+		default:
+			if seq := s.Sequence(device); seq > 0 {
+				res[device] = seq
+			}
+		}
 	}
 
-	return ver
+	return res
 }
 
 func (s *Snapshot) LocalSize() Counts {