bugfix - set backtrack events according to an *enabled* thread

bugfix - set backtrack events according to an *enabled* thread

Previously, I naively set a backtracking point for the thread that performed
the conflicting action, whether or not that thread existed at the point to
which I am backtracking. Utilize the TreeNode::is_enabled() method, plus thread
parent walking, to find the correct thread for the backtracking.

Note that this also requires a model checking event for each THREAD_CREATE
event. So add this feature.