buf_LRU_get_free_block(): Always wake up the page cleaner if needed
before exiting the inner loop.

srv_prepare_to_delete_redo_log_file():
Replace a debug assertion with a wait in debug builds.
Starting with commit 7e31a8e7
the debug assertion ut_ad(!os_aio_pending_writes())
could occasionally fail, while it would hold in core dumps of crashes.
The failure can be reproduced more easily by adding a sleep to the
write completion callback function, right before releasing to
write_slots.

srv_start(): Remove a bogus debug assertion
ut_ad(!os_aio_pending_writes()) that could fail in
mariadb-backup --prepare. In an rr replay trace, we had
buf_pool.flush_list.count==0 but write_slots->m_cache.m_pos==1
and buf_page_t::write_complete() was executing u_unlock().