ss: Have IHistoryTree#selectNextChild return a Collection instead

ss: Have IHistoryTree#selectNextChild return a Collection instead

This is more generic for multiple implementations of IHistoryTree.

From running the state system benchmarks, there was no performance impact
with this patch by returning a Collection instead of a single element.

Change-Id: I9775c856e5e30d94eba01af001495dbd61191b22
Signed-off-by: Loïc Prieur-Drevon <loic.prieurdrevon@gmail.com>
Signed-off-by: Geneviève Bastien <gbastien+lttng@versatic.net>
Reviewed-on: https://git.eclipse.org/r/78279
Reviewed-by: Alexandre Montplaisir <alexmonthy@efficios.com>
Tested-by: Alexandre Montplaisir <alexmonthy@efficios.com>
Reviewed-by: Hudson CI