client: do not shift all timerange while using ignore delay

[wdauchy]

use the minimum value to calculate the until value for the timerange used for prometheus

not sure where to put my min helper; any preference?

[iksaif]

I think the flag description should be fixed too https://github.com/criteo/graphite-remote-adapter/blob/master/main.go#L133

[wdauchy]

changes done.