coordinator: merge partial dynamic filters

[jayshrivastava]

Stack

This stack of PRs implements distributed dynamic filtering #528

1. coordinator: display dynamic filters after execution #623
2. feat: plan distributed dynamic filters #634
3. feat: forward remote dynamic filter updates to coordinator #635
4. coordinator: merge partial dynamic filters  #636 <- you are here
5. coordinator: forward merged dynamic filters to consumers #637
6. worker: apply merged dynamic filters during execution #639

Details

This PR adds machinery around merging dynamic filters in the DynamicFilterRegistry.

    register_task(stage1, task1)                                                                                              
    register_task(stage1, task2)      ─────────┐                                                                              
   register_task(stage1, task3)...             │          ┌───────────────────────┐      merge() when                         
         seal_stage(stage1)                    ├─────────▶│ DynamicFilterRegistry │───▶  - stage is sealed; and               
                                               │          └───────────────────────┘      - there's enough partial filter      
                                               │                                         updates                              
         add_partial(stage1, task1)   ─────────┘                                                                              
         add_partial(stage2, task1)                                                                                           
         add_partial(stage3, task1)                                                                                           
                                                                                                                              

The query coordinator is calling register_task on each SetPlanRequest. Concurrently, any running task can send a dynamic filter update to the registry via add_partial. The registry needs to detect when all the updates are present and merge() the partial dynamic filters. To help detect this, the coordinator is responsible for calling seal_stage once all the tasks have commenced so we know that no tasks will be added in the future.

The PR implements the above. In the next 2 PRs, we will actually forward the merged filters to consumers.

For now, we only care about completed dynamic filters. There's a follow up issue to look into incomplete dynamic filters: #665