Add a filter to short-circuit get_lastpostmodified()

[pento]

When the result of get_lastpostmodified() changes, this can cause a cache stampede on high traffic sites.

This is particularly problematic when the query is slow. See #15499.

[pento]

In 34935:

Posts: Add the pre_get_lastpostmodified filter to get_lastpostmodified().

This allows get_lastpostmodified() to be short circuited when the site wants to avoid running the occasionally slow queries in _get_last_post_time().

Fixes #34205.