[ticket/12966] Sort posts by post_id in addition to sorting by post_time

This should make sure that posts with the same post time are properly sorted
on database backends like postgres.

PHPBB3-12966
[ticket/12966] Sort by post_time and post_id where applicable

This change was not applied to the search as this would require a larger
rewrite of the search methods.

PHPBB3-12966
Merge pull request #2865 from marc1706/ticket/12966

[ticket/12966] Sort posts by post_id in addition to sorting by post_time