Field.rel is now deprecated. Rel objects have now also remote_field
attribute. This means that self == self.remote_field.remote_field.
In addition, made the Rel objects a bit more like Field objects. Still,
marked ManyToManyFields as null=True.
SQLite doesn't work with more than 1000 parameters in a single query.
The deletion code could generate queries that try to get related
objects for more than 1000 objects thus breaking the limit. Django now
splits the related object fetching into batches with at most 1000
parameters.
The tests and patch include some work done by Trac alias NiGhTTraX in
ticket #21205.
