fix: ensure WAL is atomic for each write batch (#84)

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

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

@@ -80,6 +80,12 @@ It should now store `(KeyBytes, Bytes)` and the return key type should be `KeySl
 
 Write-ahead log should now accept a key slice instead of a user key slice. When serializing and deserializing the WAL record, you should put timestamp into the WAL file and do checksum over the timestamp and all other fields you had before.
 
+The WAL format is as follows:
+
+```
+| key_len (exclude ts len) (u16) | key | ts (u64) | value_len (u16) | value | checksum (u32) |
+```
+
 **LsmStorageInner::get**
 
 Previously, we implement `get` as first probe the memtables and then scan the SSTs. Now that we change the memtable to use the new key-ts APIs, we will need to re-implement the `get` interface. The easiest way to do this is to create a merge iterator over everything we have -- memtables, immutable memtables, L0 SSTs, and other level SSTs, the same as what you have done in `scan`, except that we do a bloom filter filtering over the SSTs.