MDEV-32899 InnoDB is holding shared dict_sys.latch while waiting for FOREIGN KEY child table lock on DDL

lock_table_children(): A new function to lock all child tables of a table.
We will only hold dict_sys.latch while traversing
dict_table_t::referenced_set. To prevent a race condition with
std::set::erase() we will copy the pointers to the child tables to a
local vector. Once we have acquired references to all child tables,
we can safely release dict_sys.latch, wait for the locks, and finally
release the references.

This fixes up commit 2ca11234 (MDEV-26217)
and commit c3c53926 (MDEV-26554).