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