BUG-981: Fixed incorrect condition in ResolveDataChangeEventsTask

BUG-981: Fixed incorrect condition in ResolveDataChangeEventsTask

When we insert a new subtree, we need to make sure to call serialization
on the entire, such that any new elements which appear have their
classed attached to paths.

This reuses a previous code path, which pruned the walk tree as soon as
it encountered a node which did not have any listeners attached (thus
improving performance when we're gathering listeners).

In this specific case, though, this check is not correct: we need to
walk the path because it may be placing new data in the subtree.

At the same time add debug statements which made pinpointing this
problem possible.

Change-Id: I1687ba45817a84ea4a888496a90defc5896216c9
Signed-off-by: Tony Tkacik <ttkacik@cisco.com>
Signed-off-by: Robert Varga <rovarga@cisco.com>