Update dblink extension for parallel query.

Almost all functions provided by this extension are PARALLEL
RESTRICTED.  Mostly, that's because the leader's TCP connections won't
be shared with the workers, but in some cases like dblink_get_pkey
it's because they obtain locks which might be released early if taken
within a parallel worker.  dblink_fdw_validator probably can't be used
in a query anyway, but there would be no problem from the point of
view of parallel query if it were, so it's PARALLEL SAFE.

Andreas Karlsson