Patch to make postgres_fdw parallel-safe

This patch makes the postgres_fdw foreign data wrapper parallel-safe so that shards stored on remote servers can be accessed in parallel via a Parallel Append plan node.

Note, that the patch is experimental.

To compile and use the patch follow these instructions:

1. Get the PostgreSQL source code, for example via
   • git clone https://github.com/postgres/postgres.git --depth=1 or
   • apt-get source postgresql-11 and sudo apt-get build-dep postgresql-11
2. Apply the patch from this repository to contrib/postgres_fdw/postgres_fdw.c
3. Compile PostgreSQL
4. Copy postgres_fdw.so to /usr/lib/postgresql/11/lib/
5. Restart PostgreSQL