BUG-650: optimize subtree listener collection

This further optimizes passing of subtree listeners by retaining the
first collection of subtree listeners rather than immediately skipping
to interable concatenation.

Change-Id: I08fd63b092043ef814a47daba67744042d5c29be
Signed-off-by: Robert Varga <rovarga@cisco.com>
Merge "BUG-650: optimize subtree listener collection"