Eliminated NULL from hive_posts::root_id column. Since actual ID is not...

Eliminated NULL from hive_posts::root_id column. Since actual ID is not available yet during inserting root post, this column is updated at the end of initial-sync or processed block.
Simplified hive_posts_root_id index, to speedup list_comments_by_root queries
Optimized query used to access posts inside list_comments_by_root.
Deleted obsolete file scripts/update_hivemind_db.sql