Join-Order-Optimization Foundation

[mkleen]

Based on a first POC (https://github.com/crate/crate/tree/mkleen/join-graph) I suggest splitting the task into the following steps:

• Introduce a generic Join Plan as part of Logical Plans decoupled from the implementation to handle reordering. Once this is done the Join Plan can be converted to the NestedLoop/Hash-Join and further optimized.
• Introduce a Join Graph data structure and also the code where a Logical Plan tree can be converted to a Graph and reassembled back into a Logical Plan.
• Add a rule to the Optimizer where Join Graphs can be processed as part of the workflow.
• Start with a simple algorithm first and make sure it works end-to-end.