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 believe that one way of configuring this would be to set it globally in the scalatra bootstrap and then fine tune it per servlet.