Fix node lookup on fork-tree after a warp-sync (#11476)

* Fix node lookup on fork-tree after a warp-sync

After a warp-sync, the error condition was triggered by the absence
of the parent node of the first imported block.

The previous lookup implementation was traversing the tree using a
recursive **post-order** DFS, this technique doesn't trigger the issue.

In the last iterative implementation we were using a BFS instead.

* Added internal doc warning

* Small optimization

* Specify post-order DFS in the comment