lib/connections, lib/model: Track last connection duration (ref #7223) (#7242)

This adds a statistic to track the last connection duration per device.
It isn't used for much in this PR, but it's available for #7223 to use
in deciding how to order device connection attempts (deprioritizing
devices that just dropped our connection the last time).

lib/protocol/protocol.go

@@ -152,6 +152,7 @@ type ConnectionInfo interface {
 	Priority() int
 	String() string
 	Crypto() string
+	EstablishedAt() time.Time
 }
 
 type rawConnection struct {