Stitching Execution Plans

[michaelstaib]

Currently the stitching layer rewrites incoming queries each time. This is actually not the best approach performance-wise. We should analyze the query at the beginning and create query plan that can be cached.

In order to be able to support query plans the execution strategies need to understand an order and need to track which paths are already resolved.

Moreover, we would need to introduce a query analyzer that is able to analyze a query structure and infer from it the dependencies and execution order.

[michaelstaib]

#677