Move exec and events components to util module

The exec and events components are common components that
are used by both runtime and registry. It makes sense to
move them into the util package.

This also adds some docs to the registry module

packages/node_modules/@node-red/runtime/lib/flows/Flow.js

@@ -16,8 +16,8 @@
 
 var clone = require("clone");
 var redUtil = require("@node-red/util").util;
+const events = require("@node-red/util").events;
 var flowUtil = require("./util");
-var events = require("../events");
 const context = require('../nodes/context');
 const hooks = require("../hooks");
 
@@ -679,7 +679,6 @@ module.exports = {
         asyncMessageDelivery = !runtime.settings.runtimeSyncDelivery
         Log = runtime.log;
         Subflow = require("./Subflow");
-        Subflow.init(runtime);
     },
     create: function(parent,global,conf) {
         return new Flow(parent,global,conf);