Query_Tree:
- Query_Tree_Node:
  - remove unsused subscribers_values[]
  - update query tests

- Query_Tree refactoring to separate transactional issues from routing:
  - __query_tree_route(): only route values for all operations, returning
    subscribers_operations[]
  - __query_tree_emit() now emits for all operations and subscribers values
    retrieved by new __query_tree_route()
  - Rename _end_transaction() into __end_transaction()
  - Rename _end_source_transactions() into __end_source_transactions()
  - next step is to re-import transactional code back to pipelet.js which is
    responsible for transactions handling

- __query_tree_route#route_clear(): expressions children