juwang - update onHttpRequest() to use parsed request

The input stream from http request can be only read once. This PR is to ensure a http request is only parsed once at validation time. The parser is responsible to read stream from the request. The parsed request will be then passed as a parameter in onHttpRequest() instead of original Request. 

Once this change is merged to master, corresponding change need to be made in reef-bridge branch for handler that implement HttpHandler.