add 3.5 tests

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

mini-lsm-book/src/week3-05-txn-occ.md

@@ -1,6 +1,6 @@
 # Transaction and Optimistic Concurrency Control
 
-In this chapter, you will implement all interfaces of `Transaction`. Your implementation will maintain a private workspace for modifications inside a transaction, and commit them in batch, so that all modifications within the transaction will only be visible to the transaction itself until commit.
+In this chapter, you will implement all interfaces of `Transaction`. Your implementation will maintain a private workspace for modifications inside a transaction, and commit them in batch, so that all modifications within the transaction will only be visible to the transaction itself until commit. We only check for conflicts (i.e., serializable conflicts) when commit, and this is optimistic concurrency control.
 
 To run test cases,
 
@@ -45,6 +45,7 @@ Your commit implementation should simply collect all key-value pairs from the lo
 
 * With all the things we have implemented up to this point, does the system satisfy snapshot isolation? If not, what else do we need to do to support snapshot isolation? (Note: snapshot isolation is different from serializable snapshot isolation we will talk about in the next chapter)
 * What if the user wants to batch import data (i.e., 1TB?) If they use the transaction API to do that, will you give them some advice? Is there any opportunity to optimize for this case?
+* What is optimistic concurrency control? What would the system be like if we implement pessimistic concurrency control instead in Mini-LSM?
 
 ## Bonus Tasks