tree: rework merging inactive watchpoints from child to parent trees

[rjeczalik]

When the same chan is used to watch both watchpoints, the child and parent one, the latter will get overwritten with inactive flag coming from the child.

Quick fix would be to drop the inactive flag for this chan, however this would require maintaining temporary unique set of user chans for each dispatch to ensure single event is not dispatched multiple times to the same channel.

The proper fix would be to rework recursiveTree to not story inactive watchpoints altogether with active ones. In order to not hack the node struct, best place would be (node).Child[""].Watch, as empty-string keys are never traversed.