Avoid unnecessary buf_pool.page_hash S-latch acquisition

MDEV-15053 did not remove all unnecessary buf_pool.page_hash S-latch acquisition. There are code paths where we are holding buf_pool.mutex (which will sufficiently protect buf_pool.page_hash against changes) and unnecessarily acquire the latch. Many invocations of buf_page_hash_get_locked() can be replaced with the much simpler buf_pool.page_hash_get_low().

This causes unnecessary contention; in the worst case the thread that is holding buf_pool.mutex will become a victim of MDEV-22871, suffering from a spurious reader-reader conflict with another thread.

In many places, we are also evaluating page_id_t::fold() while holding buf_pool.mutex. Low-level functions such as buf_pool.page_hash_get_low() must get the page_id_t::fold() as a parameter.