sequence storage engine improvements

1) Implement a signed sequence table
that's a bit ugly...
select * from `sseq_-1_to_-20_step_-1`
check the "s" before seq, that's because we are using signed variables, 'BIGINT SIGNED' instead of 'BIGINT UNSIGNED'

2) implement variables sequences using vars
that's a bit ugly too...
set @variable1=1234;
set @variable2=12340;
set @variable3=1;
select * from `vseq_variable_to_variable2_step_variable3`

in this case, NaN values should report a storage engine error, and we should think if we will have only unsigned values or not