Innodb - do not allocate purge THDs during shutdown

MDEV-21751 changed the logic of so that background THD allocated when needed.
It also increased maxed out purge task during shutdown. It turns out that allocating THD when needed is not always a good idea, as THD constructor during shutdown may crash, e.g when constructor is accessing global variables, which are destroyed.

The logic needs to be changed such that THDs are only preallocated when it is safe to do never after pre-shutdown.