Support additive rollups

[rahulsom]

Currently rollups are computed as

departmentEmployeeJoin
   .computeSnapshot(id, dateOrVersion)
   .flatMap { fromIterator(it.joinedIds) }
   .flatMap { employeeQuery.computeSnapshot(it) }
   .reduce (new DepartmentSummary()) { rollup, snapshot -> ... }

There isn't much potential to do this additively.

If we can do it differently, and this works, we could try this

departmentEmployeeJoin
   .computeSnapshot(id, dateOrVersion)
   .flatMap { getAllEventsForAfterDateOrVersion(it.joinedIds, dateOrVersion) }
   .reduce (new DepartmentSummary()) {rollup, event -> ... }

The challenge is reducing

1. The number of events fetched
2. The number of computations performed
3. The number of scenarios requiring a full recompute