fix compaction split bug and add 3.1 test

Signed-off-by: Alex Chi <iskyzh@gmail.com>

mini-lsm-book/src/week3-02-snapshot-read-part-1.md

@@ -2,6 +2,18 @@
 
 During the refactor, you might need to change the signature of some functions from `&self` to `self: &Arc<Self>` as necessary.
 
-## MemTable
+## Task 1: MemTable, Write-Ahead Log, and Read Path
 
-## WAL
+Memtable store timestamp, change to scan, encode ts in wal
+
+## Task 2: Write Path
+
+assign mvcc object, take write lock, increase ts by 1
+
+## Task 3: MVCC Compaction
+
+keep all versions
+
+## Task 4: LSM Iterator
+
+return the latest version