- Import transactional code back from lib/query.js to lib/pipelet.js
  - transactions[]
  - source_transactions[]
  - __query_tree_emit()
  - __add_destination_to_transaction()
  - __end_transaction()
  - __end_source_transactions(): unused, therefore commented out after re-import

- lib/xs.js: define Dictionary(), export to XS.Dictionary

- lib/pipelet.js:
  - use Dictionary() from XS.Dictionary
  - disable traces by default

- lib/query.js: use Dictionary() from XS.Dictionary