Wire up limit and offset on show series

[pauldix]

For instance say you had 2 measurements and 3 series on each like this:

1 cpu region=uswest host=serverA
2 cpu region=uswest host=serverB
3 cpu region=uswest host=serverC
4 memory region=uswest host=serverA
5 memory region=uswest host=serverB
6 memory region=uswest host=serverC

The SHOW SERIES query will return 2 series, one for each measurement with the 3 series in each.

If they issue a query SHOW SERIES LIMIT 2 OFFSET 2 they should get back 2 series but the cpu series will only have series 3 in it and then the memory measurement with the 4 series in it