Speedup DESC queries

[za-arthur]

From #15.
The DESC query slower:

explain analyze select * from rum where tsvector @@ plainto_tsquery('simple', 'cow') order by id <=> 1000000 limit 10;
                                                                QUERY PLAN
------------------------------------------------------------------------------------------------------------------------------------------
 Limit  (cost=3.00..3.53 rows=10 width=56) (actual time=31.367..31.375 rows=10 loops=1)
   ->  Index Scan using rum_tsvector_id_idx on rum  (cost=3.00..2635.10 rows=49960 width=56) (actual time=31.365..31.371 rows=10 loops=1)
         Index Cond: (tsvector @@ '''cow'''::tsquery)
         Order By: (id <=> 1000000)
 Planning time: 0.133 ms
 Execution time: 31.427 ms

than the next ASC query:

explain analyze select * from rum where tsvector @@ plainto_tsquery('simple', 'cow') order by id <=> 0 limit 10;
                                                                QUERY PLAN
------------------------------------------------------------------------------------------------------------------------------------------
 Limit  (cost=3.00..3.53 rows=10 width=56) (actual time=18.329..18.338 rows=10 loops=1)
   ->  Index Scan using rum_tsvector_id_idx on rum  (cost=3.00..2635.10 rows=49960 width=56) (actual time=18.328..18.335 rows=10 loops=1)
         Index Cond: (tsvector @@ '''cow'''::tsquery)
         Order By: (id <=> 0)
 Planning time: 0.139 ms
 Execution time: 18.393 ms