wgengine/magicsock: reduce disco ping heartbeat aggressiveness a bit

Bigger changes coming later, but this should improve things a bit in
the meantime.

Rationale:

* 2 minutes -> 45 seconds: 2 minutes was overkill and never considered
  phones/battery at the time. It was totally arbitrary. 45 seconds is
  also arbitrary but is less than 2 minutes.

* heartbeat from 2 seconds to 3 seconds: in practice this meant two
  packets per second (2 pings and 2 pongs every 2 seconds) because the
  other side was also pinging us every 2 seconds on their own.
  That's just overkill. (see #540 too)

So in the worst case before: when we sent a single packet (say: a DNS
packet), we ended up sending 61 packets over 2 minutes: the 1 DNS
query and then then 60 disco pings (2 minutes / 2 seconds) & received
the same (1 DNS response + 60 pongs).  Now it's 15. In 1.22 we plan to
remove this whole timer-based heartbeat mechanism entirely.

The 5 seconds to 6.5 seconds change is just stretching out that
interval so you can still miss two heartbeats (other 3 + 3 seconds
would be greater than 5 seconds). This means that if your peer moves
without telling you, you can have a path out for 6.5 seconds
now instead of 5 seconds before disco finds a new one. That will also
improve in 1.22 when we start doing UDP+DERP at the same time
when confidence starts to go down on a UDP path.

Updates #3363

Change-Id: Ic2314bbdaf42edcdd7103014b775db9cf4facb47
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

wgengine/magicsock/magicsock.go

@@ -3274,7 +3274,7 @@ const (
 	// try to keep an established endpoint peering alive.
 	// It's also the idle time at which we stop doing STUN queries to
 	// keep NAT mappings alive.
-	sessionActiveTimeout = 2 * time.Minute
+	sessionActiveTimeout = 45 * time.Second
 
 	// upgradeInterval is how often we try to upgrade to a better path
 	// even if we have some non-DERP route that works.
@@ -3282,11 +3282,11 @@ const (
 
 	// heartbeatInterval is how often pings to the best UDP address
 	// are sent.
-	heartbeatInterval = 2 * time.Second
+	heartbeatInterval = 3 * time.Second
 
 	// trustUDPAddrDuration is how long we trust a UDP address as the exclusive
 	// path (without using DERP) without having heard a Pong reply.
-	trustUDPAddrDuration = 5 * time.Second
+	trustUDPAddrDuration = 6500 * time.Millisecond
 
 	// goodEnoughLatency is the latency at or under which we don't
 	// try to upgrade to a better path.