CDAP-663 Implemented healthcheck for router and auth-server.

The PR enables health check for router and auth servers independent of downstream services.  
This method is required by two handlers HttpRequestHandler (which extends SimpleChannelUpstreamHandler) and SecurityAuthenticationHttpHandler (which extends SimpleChannelHandler). Since there is no common parent for them I added it in the separate class. Another option is to create new upstream handler for serving status requests and add it in the netty pipeline before these two handlers. Please let me know your thoughts.