ss: Move selectNextChildren to CoreNode and return sequenceNumber

ss: Move selectNextChildren to CoreNode and return sequenceNumber

SelectNextChildren was only called on CoreNodes to return their
children.
Returning the SequenceNumber will allow overlapping trees to read
the nodes from disk when necessary and limit the footprint of the
queue for large queries.

Change-Id: I14ac3909bf7fb90490479d7b79cfa011c2dcb56c
Signed-off-by: Loïc Prieur-Drevon <loic.prieurdrevon@gmail.com>
Reviewed-on: https://git.eclipse.org/r/83647
Reviewed-by: Genevieve Bastien <gbastien+lttng@versatic.net>
Tested-by: Genevieve Bastien <gbastien+lttng@versatic.net>
Reviewed-by: Hudson CI