Keep what discovery heard when a round hears nothing, and fetch only changed announcements (issue 554)

A round in which no PING reached the laptop's console within the 750 ms window
was taken as the mesh's answer that nothing is there, and replaced the index:
every seat and module call was refused for five seconds at a time. Discovery
now waits up to its patience for a first PING, a round that heard nothing is
not kept, a runtime not heard this round is resolved from what it announced
last (said with its age), and a round that misses runtimes is logged once.
A PING carries a digest of what the instance serves, so an unchanged
announcement is not pulled again every round (some 880 kB to the laptop).

node-tools/internal/console/client.go

@@ -62,7 +62,7 @@ func toolKey(name string, seats Seats) string {
 // toolsOn is the flat catalogue (MESH_CONSOLE_FLAT=1): what announced itself on the bus, every
 // tool and seat verb, and the assignments with tools that did not (novox/hq ADR 0197).
 func toolsOn(conn *bus.Conn) (*Listing, error) {
-	x, err := indexOn(conn)
+	x, err := indexOn(conn, nil, nil)
 	if err != nil {
 		return nil, err
 	}