Set MySQL transaction isolation to REPEATABLE READ

Tests are passing but no performance improvements seen
on existing repository tests (including ConcurrencyTest).
Maybe more realistic workloads will behave differently.

repo/repo-sql-impl/src/main/java/com/evolveum/midpoint/repo/sql/SqlRepositoryConfiguration.java

@@ -407,7 +407,7 @@ private void computeDefaultConcurrencyParameters() {
             defaultLockForUpdateViaSql = false;
             defaultUseReadOnlyTransactions = false;        // h2 does not support "SET TRANSACTION READ ONLY" command
         } else if (isUsingMySqlCompatible()) {
-	        defaultTransactionIsolation = TransactionIsolation.SERIALIZABLE;
+	        defaultTransactionIsolation = TransactionIsolation.REPEATABLE_READ;
 	        defaultLockForUpdateViaHibernate = false;
 	        defaultLockForUpdateViaSql = true;
 	        defaultUseReadOnlyTransactions = true;