Make txn->GetState() const

Summary: makes Transaction::GetState() a const function.

Test Plan: compiles.

Reviewers: mung

Reviewed By: mung

Subscribers: andrewkr, dhruba

Differential Revision: https://reviews.facebook.net/D64929

include/rocksdb/utilities/transaction.h

@@ -416,7 +416,7 @@ class Transaction {
     LOCKS_STOLEN = 7,
   };
 
-  TransactionState GetState() { return txn_state_; }
+  TransactionState GetState() const { return txn_state_; }
   void SetState(TransactionState state) { txn_state_ = state; }
 
  protected: