log/logheap: change to POST to a URL instead of logging

It's too big to log.

tailcfg/tailcfg.go

@@ -509,10 +509,14 @@ type MapResponse struct {
 // Debug are instructions from the control server to the client
 // to adjust debug settings.
 type Debug struct {
-	// LogHeapPprof controls whether the client should logs
+	// LogHeapPprof controls whether the client should log
 	// its heap pprof data. Each true value sent from the server
 	// means that client should do one more log.
 	LogHeapPprof bool `json:",omitempty"`
+
+	// LogHeapURL is the URL to POST its heap pprof to.
+	// Empty means to not log.
+	LogHeapURL string `json:",omitempty"`
 }
 
 func (k MachineKey) String() string { return fmt.Sprintf("mkey:%x", k[:]) }