update toc for week 2

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

mini-lsm-book/src/week2-07-snacks.md

@@ -9,6 +9,16 @@ In this chapter, you will:
 * Implement the batch write interface.
 * Add checksums to the blocks, SST metadata, manifest, and WALs.
 
+## Task 1: Write Batch Interface
+
+## Task 2: Block Checksum
+
+## Task 3: SST Checksum
+
+## Task 4: WAL Checksum
+
+## Task 5: Manifest Checksum
+
 ## Test Your Understanding
 
 * Consider the case that an LSM storage engine only provides `write_batch` as the write interface (instead of single put + delete). Is it possible to implement it as follows: there is a single write thread with an mpsc channel receiver to get the changes, and all threads send write batches to the write thread. The write thread is the single point to write to the database. What are the pros/cons of this implementation? (Congrats if you do so you get BadgerDB!)