Revert "Avoid allocation storing last active time"

This reverts commit edb6929.

In that commit, active time was changed from time.Time to
Unix time in order to avoid allocations. Unfortunately, that
has the side effect of discarding the monotonic component of
time.Time, and therefore makes our code vulnerable to stepping
of the system clock.

Fixes #697

candidate_base.go

@@ -31,8 +31,8 @@ type candidateBase struct {
 
 	resolvedAddr net.Addr
 
-	lastSent     atomic.Int64
-	lastReceived atomic.Int64
+	lastSent     atomic.Value
+	lastReceived atomic.Value
 	conn         net.PacketConn
 
 	currAgent *Agent
@@ -409,27 +409,27 @@ func (c *candidateBase) String() string {
 // LastReceived returns a time.Time indicating the last time
 // this candidate was received
 func (c *candidateBase) LastReceived() time.Time {
-	if lastReceived := c.lastReceived.Load(); lastReceived != 0 {
-		return time.Unix(0, lastReceived)
+	if lastReceived, ok := c.lastReceived.Load().(time.Time); ok {
+		return lastReceived
 	}
 	return time.Time{}
 }
 
 func (c *candidateBase) setLastReceived(t time.Time) {
-	c.lastReceived.Store(t.UnixNano())
+	c.lastReceived.Store(t)
 }
 
 // LastSent returns a time.Time indicating the last time
 // this candidate was sent
 func (c *candidateBase) LastSent() time.Time {
-	if lastSent := c.lastSent.Load(); lastSent != 0 {
-		return time.Unix(0, lastSent)
+	if lastSent, ok := c.lastSent.Load().(time.Time); ok {
+		return lastSent
 	}
 	return time.Time{}
 }
 
 func (c *candidateBase) setLastSent(t time.Time) {
-	c.lastSent.Store(t.UnixNano())
+	c.lastSent.Store(t)
 }
 
 func (c *candidateBase) seen(outbound bool) {

candidate_test.go

@@ -186,15 +186,15 @@ func TestCandidateLastSent(t *testing.T) {
 	require.Equal(t, candidate.LastSent(), time.Time{})
 	now := time.Now()
 	candidate.setLastSent(now)
-	require.EqualValues(t, 0, now.Sub(candidate.LastSent()))
+	require.Equal(t, candidate.LastSent(), now)
 }
 
 func TestCandidateLastReceived(t *testing.T) {
 	candidate := candidateBase{}
 	require.Equal(t, candidate.LastReceived(), time.Time{})
 	now := time.Now()
 	candidate.setLastReceived(now)
-	require.EqualValues(t, 0, now.Sub(candidate.LastReceived()))
+	require.Equal(t, candidate.LastReceived(), now)
 }
 
 func TestCandidateFoundation(t *testing.T) {