control/controlclient: stop restarting map polls on health change

At some point we started restarting map polls on health change, but we
don't remember why. Maybe it was a desperate workaround for something.
I'm not sure it ever worked.

Rather than have a haunted graveyard, remove it.

In its place, though, and somewhat as a safety backup, send those
updates over the HTTP/2 noise channel if we have one open. Then if
there was a reason that a map poll restart would help we could do it
server-side. But mostly we can gather error stats and show
machine-level health info for debugging.

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

tailcfg/tailcfg.go

@@ -1684,6 +1684,13 @@ type SetDNSRequest struct {
 // SetDNSResponse is the response to a SetDNSRequest.
 type SetDNSResponse struct{}
 
+// HealthChangeRequest is the JSON request body type used to report
+// node health changes to https://<control>/machine/<mkey hex>/update-health.
+type HealthChangeRequest struct {
+	Subsys string // a health.Subsystem value in string form
+	Error  string // or empty if cleared
+}
+
 // SSHPolicy is the policy for how to handle incoming SSH connections
 // over Tailscale.
 type SSHPolicy struct {