list_comments api call improvements

• Improved sql for list_comments
• Removed materialized view from schema: it was not used, it required expensive refreshes
• Some calls are slow comparing to original hived calls
• by_cashout, by_update and by_author_update results are different than expected (waiting for merge from #47 (closed))
• fixed manual tests for list_comments
• Posting a comment will now update active but active is not propagating up at the moment,
• Added post object for database api post
• Added a possibility to convert amount notation to nai notation
• Other minor fixes