for sqlite back-end, add test_bulk_size_1().

get_number_of_rows() now return 1 when call:: execute(1).
rewrite sqlite3_statement_backend::fetch(int number)

test case:
    std::vector<std::string> vec_name(1);
    sql << "select name from soci_test", into(vec_name);
    assert(vec_name.size() == 1);