Build a more generic/powerful query API which writes to a store-writer.

Instead of letting the query implementation write JSON directly, let them emit
objects to a store-writer. This allows for more powerful and centralized
post-processing of the data and avoids code-duplication by moving all logic of
how to write out the data to the core. Also, this had a couple of nice side
effects and allowed for further simplication:

 - The TIMESERIES command is now handled by the front-end (based on a FETCH of
   the respective metric data); query plugin no longer have to implement this.

 - All protocol specific information is now handled by the frontend; query
   plugins no longer have to handle this (response-type).

 - Further separation of the memory-store and generic store code.