Avoid copy-pasting ruleutils files

[jasonmp85]

Because ruleutils.c's get_shard_query_def does not have external linkage, pg_shard copy-pastes ruleutils.c from the appropriate PostgreSQL release—with minimal modifications—to enable deparsing a given query back into one with shard identifiers.

These modifications are not likely to be accepted upstream as-is—they reference a shardId field we've added to deparse_context—but PostgreSQL may be open to exposing get_shard_query_def itself for external use. We'd need to:

• Devise a method that permits us to use get_shard_query_def without further modifications
• Produce a patch for PostgreSQL to expose get_shard_query_def

[jasonmp85]

I wrote a proof-of-concept for this idea here and wrote the mailing list here. No response yet, but the research and testing phases are done. Closing.