[Refactoring] Make CEvoDB mutex public

it will be locked from outside when iterating mined commitments (ref.
dash@c68b5f68aa41881c3bf36d8881b60b6855868605)

src/evo/evodb.h

@@ -30,8 +30,10 @@ class CEvoDBScopedCommitter
 
 class CEvoDB
 {
-private:
+public:
     RecursiveMutex cs;
+
+private:
     CDBWrapper db;
 
     typedef CDBTransaction<CDBWrapper, CDBBatch> RootTransaction;
@@ -50,6 +52,12 @@ class CEvoDB
         return std::make_unique<CEvoDBScopedCommitter>(*this);
     }
 
+    CurTransaction& GetCurTransaction()
+    {
+        AssertLockHeld(cs); // lock must be held from outside as long as the DB transaction is used
+        return curDBTransaction;
+    }
+
     template<typename K, typename V>
     bool Read(const K& key, V& value)
     {