Fixes in bridge.get_account_posts
[Fix] bridge_get_account_posts_by_comments/bridge_get_account_posts_by_payout/bridge_get_account_posts_by_posts properly enforce existence of given account and post (when not completely skipped)
[Fix] bridge_get_account_posts_by_payout properly pages using payout from specified post as a page border
[Fix] sum of payout and pending_payout is used in bridge_get_account_posts_by_payout instead of just payout (especially important since only not yet paid posts are picked)
added validation for observer account in get_account_posts call (when not ignored)
[Ign] removed unused pids_by_... routines from bridge_api code
[Change] Slightly changed error message on sort check for get_ranked_posts/get_account_posts to make it more informative