Keep the controller lease through a renewal the bus did not answer, until its TTL passes

A bus stall of a few seconds made a renewal time out, and the controller
called that a lost lease, exited and raised the urgent lease-lost alert,
though no other controller could take a key that still lived. Now only
another holder seen, or the key's TTL passed unrenewed, loses the lease;
new acts stop at half the TTL unrenewed and resume on the next renewal
(novox/hq issue 477).

cmd/mesh-controller/signals.go

@@ -862,7 +862,7 @@ func watchLease(f *signalFacts) []conditions.Observation {
 		}
 	}
 	if len(lost) > 0 {
-		how := "lost it: its renewal was refused or could not be made"
+		how := "lost it: another holder wrote its key, or no renewal was made within the key's age"
 		if newest.How == inventory.EpochExpired {
 			how = "stopped renewing it without giving it back, and was taken over"
 		}