wgengine: only launch pingers for peers predating the discovery protocol

Peers advertising a discovery key know how to speak the discovery
protocol and do their own heartbeats to get through NATs and keep NATs
open. No need for the pinger except for with legacy peers.

wgengine/magicsock/magicsock.go

@@ -528,6 +528,14 @@ func (c *Conn) DiscoPublicKey() tailcfg.DiscoKey {
 	return c.discoPublic
 }
 
+// PeerHasDiscoKey reports whether peer k supports discovery keys (client version 0.100.0+).
+func (c *Conn) PeerHasDiscoKey(k tailcfg.NodeKey) bool {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	_, ok := c.discoOfNode[k]
+	return ok
+}
+
 // c.mu must NOT be held.
 func (c *Conn) setNearestDERP(derpNum int) (wantDERP bool) {
 	c.mu.Lock()