Fixed #23844 -- Used topological sort for migration operation dependency resolution.

This removes the concept of equality between operations to guarantee
compatilibity with Python 3.

Python 3 requires equality to result in identical object hashes. It's
impossible to implement a unique hash that preserves equality as
operations such as field creation depend on being able to accept
arbitrary dicts that cannot be hashed reliably.

Thanks Klaas van Schelven for the original patch in
13d613f8.