Add healthcheck endpoints (#5543)

This PR adds readiness and liveness endpoints to probe Minio server
instance health. Endpoints can only be accessed without authentication
and the paths are /minio/health/live and /minio/health/ready for
liveness and readiness respectively.

The new healthcheck liveness endpoint is used for Docker healthcheck
now.

Fixes #5357
Fixes #5514

cmd/routers.go

@@ -73,6 +73,9 @@ func configureServerHandler(endpoints EndpointList) (http.Handler, error) {
 	// Add Admin router.
 	registerAdminRouter(mux)
 
+	// Add healthcheck router
+	registerHealthCheckRouter(mux)
+
 	// Register web router when its enabled.
 	if globalIsBrowserEnabled {
 		if err := registerWebRouter(mux); err != nil {