ipn/localapi: refresh ACME certs in background two weeks in advance

Updates #1235

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

ipn/localapi/localapi.go

@@ -57,12 +57,6 @@ type Handler struct {
 	b            *ipnlocal.LocalBackend
 	logf         logger.Logf
 	backendLogID string
-
-	// certMu guards all cert/ACME operations, so concurrent
-	// requests for certs don't slam ACME. The first will go
-	// through and populate the on-disk cache and the rest should
-	// use that.
-	certMu sync.Mutex
 }
 
 func (h *Handler) ServeHTTP(w http.ResponseWriter, r *http.Request) {