simplify TrackMessageSizeInterceptor configuration

I figured a better way of doing this... I can rebase if you agree this is better.
The previous version let the servlet configuration override `trackMessageSize` method - i.e if `trackMessageSize` is intentionally set to false and the servlet configuration is true then the interceptor will be nonetheless added.
I'm believe that one way of configuring this would be to set it globally in the scalatra bootstrap and then fine tune it per servlet.