Currently we have `Pager::update_dirty_loaded_page_in_cache` which does
exactly what you would expect, but `DumbLruPageCache::_insert` method
with `ignore_existing` set to true, totally ignores the previous entry
and leaks the memory.
I really want to get #2885 finished and through because of the perf, but
I ran into this when inspecting it for correctness changes
Closes#2892