* IDs are allocated by the parent responsible for contructing a new
  child, using ALLOCATE_ID to the master as necessary to allocate new ID
  ranges.

* ADD_ROUTE is sent up the tree rather than down. This permits
  construction of the new context to complete concurrent to parent
  contexts learning about its existence. Since all streams are strictly
  ordered, it's not possible for any parent to observe messages from the
  new context prior to arrival of an ADD_ROUTE from the parent notifying
  of its existence.

  If the new context, for example, implements an Ansible async task, its
  parent can start executing that without waiting for any synchronous
  confirmation from any parent or the master.

* Since routes propagate up, it's no longer possible for a plain
  non-parent child to ever receive ADD_ROUTE, so that code can be moved
  out of core.py and into parent.py (-0.2kb compressed).

* Add a .routes attribute to parent.Stream, and respond to disconnection
  signal on the stream by propagating DEL_ROUTE for any ADD_ROUTE ever
  received from that stream.

* Centralize route management in a new parent.RouteMonitor class