Rather than flushing the post right away only to find out that there
were validation errors, try to postpone flushing for as long as
possible.
The previous behavior has led to too eager spending of post IDs - each
flush calls nextval(post_id_seq), and postgres sequences are not
affected by transaction rollbacks, so each erroneous post creation
discarded a post ID, which has led to gaps in post IDs.
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