added task and job for AggregateStatistics

this is part of the fix for 907

the idea is we stop running the AggregateStatistics proc from the Gallery and we start running it from the worker

we are doing this because the proc has concurrency problems, however, fixing those directly would incur a performance penalty on the home page so we are simply moving the work to the worker when in azure.

(we made the change to the Gallery configurable so you can optional run it there if you like - for example if you have a single instance of the Gallery running.)