Make btr_cur_latch_leaves not latch the sibling pages it prefetches
under fake changes transactions.

Accept unlatched pages in btr_page_get_next if called from a fake
changes transaction.
Simplify fake changes handling in row_ins_clust_index_entry_low.
Assert that btr_cur_pessimistic_insert cannot be called with
uninitialized cursor tree height instead of handling that
case. Simplify the fake changes bits to reduce the upstream delta.
Make lock_trx_has_rec_x_lock aware of fake changes and re-enable its
calls.