MVCC: SQL tx operations and DDL inside tx block

DDL and special tx (BEGIN, COMMIT) operations does not behave well inside explicit tx started using Java API. We should define how such operations behave inside tx or forbid them inside tx. See test SqlTransactionsCommandsWithMvccEnabledSelfTest#testSqlOperationsWithinNonSqlTransaction.
Here is an example of problematic construction:

try (Transaction tx = node.transactions().txStart(PESSIMISTIC, SERIALIZABLE)) {
    cache.put(1, 1);
    cache.query(new SqlFieldsQuery("commit"));
    cache.put(2, 2);

    tx.commit();
}