Slow update_posts_rshares

• It's still slow in after-initial-sync phase Tested 12efa972 in both U18 & U20 environments (PostgreSQL 10 & PostgreSQL 12 respectively)
  It takes up to 32298s (U20 setup), and on my reference machine it's 24849s
  (please note that envs are with very different hardware setup so they can't be directly compared)
  It's still improvement from #123 (closed) times when it took 40442s (on roughly 50M instead of 57M)
• It's unexpectedly slow in certain cases at live sync On U20 environment, it's
  SELECT update_posts_rshares(57208020, 57208020) executed in: 0.1749 s
  whereas on refernce machine it's
  SELECT update_posts_rshares(57208020, 57208020) executed in: 0.0027 s