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/testutils/testutils.go

@@ -10,6 +10,7 @@ import (
 	"errors"
 	"net"
 	"sync"
+	"time"
 )
 
 var ErrClosed = errors.New("closed")
@@ -90,6 +91,10 @@ func (f *FakeConnectionInfo) String() string {
 	return ""
 }
 
+func (f *FakeConnectionInfo) EstablishedAt() time.Time {
+	return time.Time{}
+}
+
 type FakeAddr struct{}
 
 func (FakeAddr) Network() string {