Reject impressions with histogramIndex >= maximum histogram size

[apasel422]

Since they will never be able to fill a histogram.

Fixes #197

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[martinthomson]

It occurs to me that there is a risk that the limit is dictated by the aggregation service. That's fine on the conversion side, because we have the details of the chosen aggregation service and the associated method available. That's not going to be true on the impression-saving side.

This can probably only catch the case where the index exceeds the maximum across all aggregation services (or their associated processes). WDYT?

[csharrison]

I think that should be a separate limit and error. It is natural for the client to impose a different maximum which covers both impression and conversion.

[martinthomson]

That makes sense to me. @apasel422, do you agree? If so, this will need to be separate concepts, though they might benefit from an informational pointer.

[apasel422]

@apasel422, do you agree?

I agree that it makes sense to consider the limit imposed by aggregation services. I'll update this PR with the separate check on the conversion side.