Partitioning of reports

[martinthomson]

When enforcing anti-replay bounds, or even just in how tasks are constructed, it might be useful for a site to be able to segregate their aggregation tasks into different partitions. This likely results in greatly reduced costs for aggregators, which could be passed on to the sites that use aggregation.

A partitioning key is only useful if both the reports and the aggregation tasks can be bound to a partition. For the reports, we'll need a way to specify that.