Dependency-aware node ordering for replay

[ChrisKeefe]

In order to successfully replay an analysis, we must order Actions such that all dependencies of an Action are satisfied before that Action may be run - beginning with nodes of in-degree 0, and ending with nodes of out-degree 0.

I think a topological sort will do this for us.

Topological sorts are not by definition unique, so we may get varying sort results as things change. (e.g this python version change -> sort order change). lexical_topological_sort will let us impose a more nuanced ordering, at the cost of a little bit of complexity. This commit from the same issue is the best example I've found to date of what a key function might look like.

[ChrisKeefe]

Questions:

• How does topological_sort handle disconnected graphs?
• Is this implementation working from optimal starting nodes (all nodes with no in-edges)? (If not, we may be running multiple searches, and cost might merit a DIY solution at some point)