When there is a huge transaction in the undo log, the purge threads
may get stuck in trx_purge_attach_undo_recs() for a long time,
causing the server to hang on a normal shutdown (innodb_fast_shutdown>0).

Apparently the innodb_purge_batch_size does not work correctly, or the
n_pages_handled is not being incremented correctly. We do not fix that
for now, but we will instead check if shutdown has been initiated,
allowing the purge threads to shut down without delays.