Before this PR, DataNode considered auto-flush a valid flush
complete. It's wrong. So I open this PR to correct this behaviour
in DataNode.

Now binlog paths from auto-flush will be buffered in replica,
waiting until the manul flush to save into etcd all together.

See also: #5220, #5268
A follow up job of #5271 
Signed-off-by: Nyangxuan <xuan.yang@zilliz.com>