Made MigrationGraph forwards_plan() and backwards_plan() fall back to an
iterative approach in case the recursive approach exceeds the recursion
depth limit.
Switched from an adjancency list and uncached, iterative depth-first
search to a Node-based design with direct parent/child links and a
cached, recursive depth-first search. With this change, calculating
a migration plan for a large graph takes several seconds instead of
several hours.
Marked test `migrations.test_graph.GraphTests.test_dfs` as an expected
failure due to reaching the maximum recursion depth.
Previous algo only worked if the first item was a part of the loop,
and you would get an infinite loop otherwise (see test).
To fix this, it was much easier to do a pre-pass.
A bonus is that you now get an error message that actually helps you debug
the dependency problem.
Yoong Kang Lim
Simon Charette
Markus Holtermann
Marten Kenbeek
Tim Graham
Josh Schneier
Luke Plant
Andrew Godwin
Markus Bertheau
Ben Reilly
Raffaele Salmaso