Support flexible report grouping

[cloudflare]

Support flexible report grouping

[nakatsuka-y]

Here are plans for supporting "fixed-sized chunks" in Daphne:

1. Add QueryType, ChunkQueryType, ChunkQuery, and Query in messages.rs
2. Add QueryConfig in lib.rs and add QueryConfig element to TaskConfig
3. Change Leader functionality to store received Reports in chunks if QueryType in QueryConfig is fixed-chunk. Reports should be stored in chunks associated to chunk ID (TODO: Should the chunk ID be random?)
4. CollectReq sent by the Collector would be changed to include a Query and the CollectResp must be changed to include a chunk ID if the QueryType is fixed-chunk.
5. Replace all functionality that depends on BatchIntervals in Daphne with Query to reflect the changes (including messages that use batch_interval)
6. Reflect changes to how different entities in Daphne stores data. Especially Report store and Agg share store store state associating them with batch windows. The implementation would be different from MockAggregator and Daphne Workers.
7. Implement methods that check the query received by the Collector (batch boundaries, number of reports requested, and batch overlap)
8. Implement test cases where the Collector sends different types of queries.

[cjpatton]

This will be addressed in #100