Set: remove operations order counstainst: remove before update before add, details:
  - Removing operation order countrainst allows easier horizontal distribution and charding of pipelet
  - implement anti-set (b) to menorize removes happening before adds
  - _index_of() allows lookups in anti-set
  - make_index_of() can generate additional function _index_of() on anti-set
  - add(): check if values are in anti-set before adding
  - remove(): add to anti-set if not found
  - update(): if previous value not found, add to anti-set and add new value to set
Aggregates: update data set on changes to measures and dimensions:
  - use Pipelet.build and Pipelet.subclass
  - use assynchronous fetch_all()
  - build merge dynamically
  - build_group and build_reduce_groups: call done function to allow other changes
  - implement dimensions_changed() and measures_changed() to update other functions
    and reprocess aggregates on measures or dimensions change
XS Tests: new tests for unordered operations on sets; try one assynchronous test using callback on fetch_all