cmd/tailscale/cli: use optimistic concurrency control on SetServeConfig

This PR uses the etag/if-match pattern to ensure multiple calls
to SetServeConfig are synchronized. It currently errors out and
asks the user to retry but we can add occ retries as a follow up.

Updates #8489

Signed-off-by: Marwan Sulaiman <marwan@tailscale.com>

cmd/tailscale/cli/serve_dev.go

@@ -275,6 +275,9 @@ func (e *serveEnv) runServeCombined(subcmd serveMode) execFunc {
 		}
 
 		if err := e.lc.SetServeConfig(ctx, parentSC); err != nil {
+			if tailscale.IsPreconditionsFailedError(err) {
+				fmt.Fprintln(os.Stderr, "Another client is changing the serve config; please try again.")
+			}
 			return err
 		}