make GenericBackrefExtension smarter about sparse collections

[jek]

When a lazy=None backref'd relation is built, push down a 'sparse collection' (e.g.) flag into the GenericBackrefExtension. Use this knowledge when handling a set()- the 'old value' may not exist in the collection, and that's not an error. Probably add a 'check first' option to the attribute's remove() to implement that logic. May also require adding an (optional?) 'contains' hook to the collections api.

A refinement of #813 / r3613.

[Mike Bayer]

there's no error case for lazy=None right now and no need for what this ticket is suggesting.

[Mike Bayer]

Removing milestone: 0.5.xx (automated comment)