feat: debug middleware enter log on every request (#210)

Usage:

```bash
DEBUG=egg-core:middleware egg-bin start
```

Author: fengmk2

lib/loader/mixin/middleware.js

@@ -78,6 +78,10 @@ module.exports = {
       // middlewares support options.enable, options.ignore and options.match
       mw = wrapMiddleware(mw, options);
       if (mw) {
+        if (debug.enabled) {
+          // show mw debug log on every request
+          mw = debugWrapper(mw);
+        }
         app.use(mw);
         debug('Use middleware: %s with options: %j', name, options);
         this.options.logger.info('[egg:loader] Use middleware: %s', name);
@@ -110,3 +114,12 @@ function wrapMiddleware(mw, options) {
   fn._name = mw._name + 'middlewareWrapper';
   return fn;
 }
+
+function debugWrapper(mw) {
+  const fn = (ctx, next) => {
+    debug('[%s %s] enter middleware: %s', ctx.method, ctx.url, mw._name);
+    return mw(ctx, next);
+  };
+  fn._name = mw._name + 'DebugWrapper';
+  return fn;
+}