add the ability to check the memory cost of a session for an app

[aihazm]

As a developer I want to be able to check the amount of memory a session will take before publishing my apps

[wennmo]

The static memory usage for the app/data model is present in the meta data. However additional sessions against the same app reuses the memory cache, so the average memory footprint per session will probably be lower. Probably hard to determine such a metric without profiling the app.