posts_cache invalid data

[roadscape]

Somehow hive_posts_cache ended up with a large number of rows where id's did not match proper records. hive_posts has strict constraints so fortunately this is easy to detect; however it should never ever happen.

[roadscape]

These should always return 0:

SELECT count(*)
  FROM hive_posts hp
  JOIN hive_posts_cache hpc ON post_id = id
 WHERE hp.author != hpc.author

SELECT count(*)
  FROM hive_posts hp
  JOIN hive_posts_cache hpc ON post_id = id
 WHERE hp.permlink != hpc.permlink

[roadscape]

Changes:

1. never rely on hpc for id map: https://github.com/steemit/hivemind/commit/0153e8c61cf635ed99dec334c8a152cd44ac8e06#diff-edb401a6f9d72cecf7670d6d5d054e8fR145
2. paranoid checks on
   • updating hpc id map (same entry should never have diff pid)
     • https://github.com/steemit/hivemind/blob/7182f7dedab90a9b9a07cc7bfcf0a1056acdaadd/hive/indexer/cached_post.py#L89
   • fetching content (enforcing expected id's, in expected order)
     • https://github.com/steemit/hivemind/blob/7182f7dedab90a9b9a07cc7bfcf0a1056acdaadd/hive/indexer/cached_post.py#L226

Most likely the cause was a bug during rewrites/testing late last week. The extra assertions haven't been tripped so whatever the issue was seems to have been a temporary one. Leaving them in place for the time being.