Defining a 10x granularity for a search widget pivot, can result in redundant timestamps

As you can see in the screenshot, it is possible to see the same timestamp multiple times with different related values in a data table.


## Expected Behavior
Timestamps should be uniq. I guess due to the high granularity, 1 second is not the smallest unit.

## Steps to Reproduce (for bugs)
1. Open the search page
2. Define a relative time range of 5 minutes
3. Create a data table aggregation
4. Define a timestamp row pivot
5. Set the automatic interval granularity to "10x"

* Graylog Version: 3.2.0-beta.4-SNAPSHOT