Implement worker-to-worker shard copy UDF

[jasonmp85]

The master node already has a UDF to repair an existing placement, but it requires a corresponding UDF on the worker side to actually copy the data between workers. This PR implements that UDF.

Code Review Tasks

• [x] Remove copy_shard files and consolidate code in repair_shards files
• [x] Improve error message style
• [x] Reorder CopyDataFromFinalizedPlacement arguments
• [x] Consider use of ExecuteRemoteCommand
• [x] Investigate need for heap_open/heap_open
• [x] Check whether ExecMaterializeSlot creates a copy
• [x] Change ExecuteRemoteCommand to permit PGRES_TUPLES_OK
• [x] Call ExecuteRemoteCommandList instead of handling manually
• [x] Delay relationKind assignment until right before if block

[jasonmp85]

I added a task list to this review. I've (locally) done most of the small changes but will check off each of the larger ones as they're addressed.

[jasonmp85]

I've addressed all of your feedback. Next round!