thread: fix parent/child relationships

thread: fix parent/child relationships

The ordering change in add_child is critical if $self == $parent
as the {children} hash was lost before this change.

has_descendent can be simplified by walking upwards from the child
instead of downwards from the parent.

This fixes threading regressions introduced in
commit 30100c46326e2eac275e0af13116636701d2537e
("thread: use hash + array instead of hand-rolled linked list")