BUG-1493: do not instantiate HashSet

This HashSet is used to track processed nodes -- but the parent node is
already giving us a method, which checks if it contains a particular
node ID. Let's use that instead of maintaining the set.

Based on traces, this should speed the datastore up by about 4.8%.

Change-Id: I09e1135f51c8967b9fe8c520a0e166918472de7d
Signed-off-by: Robert Varga <rovarga@cisco.com>
Merge "BUG-1493: do not instantiate HashSet"