Normally, buf_pool.flush_list must be sorted by
buf_page_t::oldest_modification, so that log_checkpoint()
can choose MIN(oldest_modification) as the checkpoint LSN.

During recovery, buf_pool.flush_rbt used to guarantee the
ordering. However, we can allow the buf_pool.flush_list to
be in an arbitrary order during recovery, and simply ensure
that it is in the correct order by the time a log checkpoint
needs to be executed.

recv_sys_t::apply(): To keep it simple, we will always flush the
buffer pool at the end of each batch.

Note that log_checkpoint() will invoke recv_sys_t::apply() in case
a checkpoint is initiated during the last batch of recovery,
when we already allow writes to data pages and the redo log.

Reviewed by: Vladislav Vaintroub