fix splitting of queries when step is larger than split interval

[sandeepsukhani]

What this PR does / why we need it:
The query split interval code is written with the assumption of step always being smaller than the split interval. This causes the following problems when step is larger than the split interval:

1. The first split is set to the same start and end time here since we find the next interval and then try to align the difference by step which would always be of smaller value than step which then makes the end time to be set to the start time.
2. We end up with a gap in query interval because the start time of the second split would be set to + here, but the previous query did not cover any interval due to it being set to same start and end time.

Checklist

• Tests updated

[owen-d]

NIce catch

[kavirajk]

LGTM