Timeseries: Avoid collisions with columns named 'timestamp'

[Algunenano]

PR to avoid name clashes when the dataset column in named timestamp which trigger and invalid GROUP BY so you could get several bins with the same number. For example:

{"aggregation":"decade","offset":0,"timestamp_start":631152000,"bin_width":315576000,"bins_count":3,"bins_start":631152000,"nulls":0,"bins":[{"bin":0,"min":631152000,"max":631152000,"avg":631152000,"freq":123,"timestamp":631152000},{"bin":0,"min":631152000,"max":631152000,"avg":631152000,"freq":119,"timestamp":631152000},{"bin":0,"min":631152000,"max":631152000,"avg":631152000,"freq":122,"timestamp":631152000},{"bin":0,"min":631152000,"max":631152000,"avg":631152000,"freq":120,"timestamp":631152000},{"bin":1,"min":946684800,"max":946684800,"avg":946684800,"freq":116,"timestamp":946684800},{"bin":1,"min":946684800,"max":946684800,"avg":946684800,"freq":127,"timestamp":946684800},{"bin":1,"min":946684800,"max":946684800,"avg":946684800,"freq":120,"timestamp":946684800},{"bin":1,"min":946684800,"max":946684800,"avg":946684800,"freq":129,"timestamp":946684800},{"bin":1,"min":946684800,"max":946684800,"avg":946684800,"freq":126,"timestamp":946684800},{"bin":1,"min":946684800,"max":946684800,"avg":946684800,"freq":124,"timestamp":946684800},{"bin":1,"min":946684800,"max":946684800,"avg":946684800,"freq":125,"timestamp":946684800},{"bin":2,"min":1262304000,"max":1262304000,"avg":1262304000,"freq":116,"timestamp":1262304000},{"bin":2,"min":1262304000,"max":1262304000,"avg":1262304000,"freq":110,"timestamp":1262304000},{"bin":2,"min":1262304000,"max":1262304000,"avg":1262304000,"freq":112,"timestamp":1262304000},{"bin":2,"min":1262304000,"max":1262304000,"avg":1262304000,"freq":117,"timestamp":1262304000},{"bin":2,"min":1262304000,"max":1262304000,"avg":1262304000,"freq":113,"timestamp":1262304000}],"type":"histogram"}

Now:

{"aggregation":"decade","offset":0,"timestamp_start":631152000,"bin_width":315576000,"bins_count":3,"bins_start":631152000,"nulls":0,"bins":[{"bin":0,"min":631152000,"max":631152000,"avg":631152000,"freq":607,"timestamp":631152000},{"bin":1,"min":946684800,"max":946684800,"avg":946684800,"freq":1241,"timestamp":946684800},{"bin":2,"min":1262304000,"max":1262304000,"avg":1262304000,"freq":568,"timestamp":1262304000}],"type":"histogram"}

Fixes #827

[Algunenano]

In the numeric histograms we there could be issues with columns named 'bins' so I've applied the same principle to avoid it.

Should fix https://github.com/CartoDB/support/issues/1181

[simon-contreras-deel]

I don't like to group by an ordinal number of an output column because it is harder to understand and maintain.

But as you said in #827 (comment) changing the name of the output column could have the same problem in the future.

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