3scale batching policy: improve timestamp resolution

[davidor]

The 3scale batching policy reports all the batched transactions with the same timestamp. This shouldn't be a problem in practice because the batching period is not supposed to be very long, a few seconds or minutes at most.

In the event of a downtime in the 3scale backend, reports could be accumulated for minutes and when 3scale becomes available again, all the batched reports would be reported with the same timestamp.

This could be a problem because the stats shown in the 3scale UI could not be very accurate.

In order to avoid that, we could group reports with the same credentials by the minute. A finer granularity would not make sense because there are not any charts in the 3scale UI with a resolution < 1 minute.