trx_purge_truncate_history(): In case of an apparent conflict with
buf_pool_t::release_freed_page(), always momentarily acquire
buf_pool.mutex, to let the thread that is holding it and waiting for
buf_pool.flush_list_mutex to proceed.

The fix in commit c410f7aa was
insufficient, because simple yielding would not necessarily let
the conflicting thread to acquire buf_pool.flush_list_mutex.

This hang was reported and the fix tested by Axel Schwenke.