Implement xdbc_fdw parallel functions

[Hyrikan]

• Implement basic fdw functions
• Implement IsForeignScanParallelSafe in fdw, which just returns true now :P
• implement EstimateDSMForeignScan in fdw, for that get to know, how and where shared memory is needed in the fdw.
• implement InitializeDSMForeignScan in fdw, which prepares the shared memory. POINTERS FORBIDDEN!
• ReInitializeDSMForeignScan needed? probably not for now
• implement InitializeWorkerForeignScan in fdw, here initialize the state of one worker based on the shared state and all structures needed for one worker.
• implement IterateForeignScan in fdw, which gets called by each worker in parallel. So it has to use the private and shared state to determine its result tuple.