Properly solve the backtracking issue from 10/8

[mkazin]

When I was working on traversing the graph I ran into some issues in the recursion. First among these was recursing over already-queried Players (who had also been appended into the player_cache). My naive solution to this was first to add a second parameter into the recursion function to allow checking for backtracking. I then determined I could simplify this by using player_cache. I now believe this may have been a mistake. The reason: a Player skipped due to backtracking may require different handling than one due to graph_depth.

Example:

• Suppose the following list of relationships: Alice -> Bob, Debra Bob -> Carl Carl -> Debra Debra -> Eustice

• Using the graph traversal system I currently employ, after three recursions the Bob branch would be: Alice -> Bob -> Carl -> Debra

• If the max_depth was set to 3, we would not query for Debra's relationships, but we would add her to the cache.

• The problem occurs when we exit Bob's recursion stack, return to iterating over Alice's friends, and re-encounter Debra.

• At this stage, Debra is skipped due to being in the cache. So despite being well within the specified graph depth, we would never build up the other branch under Alice: Alice -> Debra -> Eustice

[mkazin]

Reminder to self: the keyword here was "due to graph_depth". The unit test I wrote yesterday had a high depth. Reduce that to get the expected test failure.