Add support for COPY

This capability was introduced to PostgreSQL in commit
3d956d9562aa4811b5eaaaf5314d361c61be2ae0 in v11.

Attempting COPY on a foreign table crashed PostgreSQL before this patch,
as reported by "jkldv" in #309.

This patch is still incomplete:
- too little testing
- crashes if there is an AFTER INSERT trigger
- no regression tests yet