Fetching more than all results in router executor queries fails spuriously

[anarazel]

As reported by @onderkalaci in #578 (not really related to that issue), we currently don't handle fetching from a cursor that already returned all its rows correctly.

CREATE TABLE multiple_hash
(
    category text NOT NULL,
    data text NOT NULL 
);

INSERT INTO multiple_hash VALUES ('1', '1');
INSERT INTO multiple_hash VALUES ('1', '2');
INSERT INTO multiple_hash VALUES ('1', '3');
INSERT INTO multiple_hash VALUES ('1', '4');
INSERT INTO multiple_hash VALUES ('1', '5');
INSERT INTO multiple_hash VALUES ('1', '6');
INSERT INTO multiple_hash VALUES ('1', '7');

BEGIN;
DECLARE test_cursor CURSOR FOR 
    SELECT *
        FROM multiple_hash
        WHERE category = '0'
        ORDER BY category;

-- the same error exists on both `master` and `feature/returning` branches:
FETCH FORWARD 4 test_cursor;
FETCH FORWARD 4 test_cursor;
ERROR:  scan directions other than forward scans are unsupported

should be trivial to fix, by accepting NoMovementDirection as well, and returning without executing the query.