Enfore TransactionIsolationLevel.READ_COMMITTED

By default, all supported databases but MySQL are using this
isolation level. However, without setting this property, gap
locks may appear in MySQL :

https://dev.mysql.com/doc/refman/5.7/en/innodb-locking.html#innodb-gap-locks

server/sonar-db-dao/src/main/java/org/sonar/db/MyBatis.java

@@ -28,6 +28,7 @@ import org.apache.ibatis.session.ExecutorType;
 import org.apache.ibatis.session.SqlSession;
 import org.apache.ibatis.session.SqlSessionFactory;
 import org.apache.ibatis.session.SqlSessionFactoryBuilder;
+import org.apache.ibatis.session.TransactionIsolationLevel;
 import org.sonar.api.Startable;
 import org.sonar.db.ce.CeActivityMapper;
 import org.sonar.db.ce.CeQueueMapper;
@@ -267,10 +268,10 @@ public class MyBatis implements Startable {
 
   public DbSession openSession(boolean batch) {
     if (batch) {
-      SqlSession session = sessionFactory.openSession(ExecutorType.BATCH);
+      SqlSession session = sessionFactory.openSession(ExecutorType.BATCH, TransactionIsolationLevel.READ_COMMITTED);
       return new BatchSession(session);
     }
-    SqlSession session = sessionFactory.openSession(ExecutorType.REUSE);
+    SqlSession session = sessionFactory.openSession(ExecutorType.REUSE, TransactionIsolationLevel.READ_COMMITTED);
     return new DbSessionImpl(session);
   }