Add method of gettimestamp for transaction (#303)

* add interface of timestamp

Signed-off-by: Little-Wallace <bupt2013211450@gmail.com>

* fix comment

Signed-off-by: Little-Wallace <bupt2013211450@gmail.com>

* Update src/transaction/transaction.rs

Co-authored-by: Andy Lok <andylokandy@hotmail.com>
Signed-off-by: Little-Wallace <bupt2013211450@gmail.com>

Co-authored-by: Andy Lok <andylokandy@hotmail.com>
Co-authored-by: Ziqian Qin <ekexium@gmail.com>

src/transaction/client.rs

@@ -183,7 +183,9 @@ impl Client {
 
     /// Request garbage collection (GC) of the TiKV cluster.
     ///
-    /// GC deletes MVCC records whose timestamp is lower than the given `safepoint`.
+    /// GC deletes MVCC records whose timestamp is lower than the given `safepoint`. We must guarantee
+    ///  that all transactions started before this timestamp had committed. We can keep an active
+    /// transaction list in application to decide which is the minimal start timestamp of them.
     ///
     /// For each key, the last mutation record (unless it's a deletion) before `safepoint` is retained.
     ///

src/transaction/transaction.rs

@@ -635,6 +635,11 @@ impl<PdC: PdClient> Transaction<PdC> {
         res
     }
 
+    /// Get the start timestamp of this transaction.
+    pub fn start_timestamp(&self) -> Timestamp {
+        self.timestamp.clone()
+    }
+
     /// Send a heart beat message to keep the transaction alive on the server and update its TTL.
     ///
     /// Returns the TTL set on the transaction's locks by TiKV.