possible fix for very slow update update_posts_rshares function

During investigations it showed up that when  the db which is synced
almost to blockchain head, postgres generate plans which use 'index
scans' for hive_votes table and 'merge join', what is not efficient because
after initial sync every row in the hive_votes table must be checked during executions
of 'update_posts_rshares' procedure. It was tested that much better
performance is reached when  seq scans are allowed and postgres
generates hash joins.