payout sweep performance degrades quickly

[roadscape]

Breaking this task out of #16 (index state management)
Also related to #52 (db performance)

The post payout sweep query quickly becomes the heaviest by far. Need move the payout sweep to in-memory payout queue and/or examine db perf. Currently it's the only read query against hive_posts_cache in the indexer, and may be a sign of performance issues the API layer will face.

The issue with the table is that it's large, wide, and every update is a full one. #16 has other subtasks ('events which affect payout/votes vs. content of posts', 'flush inserts asap, edits over an n-block period') which will keep writes smaller and help prevent fragmentation. Besides this, may need to look into partitioning the tables, definitely tune autovac, and more.

[DEBUG] total SQL time: 1800s
 85.2%  1533237ms    521.33avg     2941x -- SELECT post_id, author, permlink FROM hive_posts_cache WHERE is_paidout = '0' AND payout_at <= :date
  5.3%   94818ms      1.01avg    93974x -- UPDATE hive_posts_cache SET author = :author, permlink = :permlink, category = :category, depth = :depth, children = :children, title = :title, preview = :preview, body = :body, im
  1.5%   26630ms      0.57avg    46829x -- UPDATE hive_accounts SET proxy = :proxy, post_count = :post_count, reputation = :reputation, proxy_weight = :proxy_weight, vote_weight = :vote_weight, kb_used = :kb_used, active_at
  1.4%   25183ms      0.35avg    72779x -- SELECT tag FROM hive_post_tags WHERE post_id = :id
  1.2%   22499ms      0.56avg    40379x -- SELECT id FROM hive_posts WHERE author = :a AND permlink = :p
  1.1%   19431ms      0.73avg    26566x -- INSERT INTO hive_posts_cache (post_id, author, permlink, category, depth, children, title, preview, body, img_url, payout, promoted, payout_at, updated_at, created_at, rshares, vot

[roadscape]

Moving to index-only scan on hpc and pulling the rest of the data from hp is holding up well.. after 12 hrs the total cost for each sweep averages 3.44ms.

  2.7%   44330ms      2.99avg    14819x -- SELECT post_id FROM hive_posts_cache WHERE is_paidout = '0' AND payout_at <= :date
  0.4%    6603ms      0.45avg    14770x -- SELECT id, author, permlink FROM hive_posts WHERE id IN :ids