Add design for datanode flowgraph recovery (#5562)

Signed-off-by: yangxuan <xuan.yang@zilliz.com>

docs/design_docs/datanode_flowgraph_recovery_design_0604_2021.md

+# DataNode Flowgraph Recovery Design
+
+update: 6.4.2021, by [Goose](https://github.com/XuanYang-cn)
+
+## 1. Common Sense
+A. 1 message stream to 1 vchannel, so there are 1 start position and 1 end position in 1 message pack
+B. Only when datanode flushes, datanode will update every segment's position
+An optimization: update position of
+  - a. Current flushing segment 
+  - b. StartPosition of segments never been flushed.
+C. DataNode auto-flush is a valid flush.
+D. DDL messages are now in DML Vchannels.
+
+## 2. Flowgraph Recovery
+### A. Save checkpoints
+When a flowgraph flushes a segment, we need to save these things:
+- current segment's binlog paths,
+- current segment positions,
+- all other segments' current positions from replica (If a segment hasn't been flushed, save the position when datanode first meet it.)
+
+Whether save successfully:
+- If successed, flowgraph updates all segments' position to replica
+- If not
+    - For a grpc failure( this failure will appear after many times retry internally), crush itself.
+    - For a normal failure, retry save 10 times, if fail still, crush itself. 
+
+### B. Recovery from a set of checkpoints
+1. We need all positions of all segments in this vchannel `p1, p2, ... pn`
+
+    [TBD] A design of WatchDmChannelReq
+
+2. We want to filter msgPacks based on these positions.
+
+![recovery](graphs/flowgraph_recovery_design.png)
+
+Supposing we have segment `s1, s2, s3`, corresponding position `p1, p2, p3`
+  - Sort positions in reverse order `p3, p2, p1`
+  - Get segments dup range time: `s3 ( p3 > mp_px > p1)`, `s2 (p2 > mp_px > p1)`, `s1(zero)`
+  - Seek from the earliest, in this example `p1`
+  - Then for every msgPack after seeking `p1`,  the pseudocode:
+
+```go
+const filter_threshold = recovery_time
+// mp means msgPack
+for mp := seeking(p1) {
+    if mp.position.endtime < filter_threshod {  
+        if mp.position < p3 {
+            filter s3
+        }
+        if mp.position < p2 {
+            filter s2
+        }
+    }
+}
+```
+
+
+