support for post-loading of relationships via IN loading

[Mike Bayer]

this would be a new loader "selectin" for relationships that does "SELECT related FROM parent JOIN related WHERE parent.id IN (...loaded ids...)" and a similar idea for joined inheritance. The loader fires within a new section of the instance load that is after each batch of objects is loaded. because it's per-batch it would also work with yield-per.

[Mike Bayer]

initial POC: https://gerrit.sqlalchemy.org/352

[Mike Bayer]

joined inheritance is at #3948.

[Mike Bayer]

Add selectin loading

Adding a new kind of relationship loader that is a cross between the "immediateload" and the "subquery" eager loader, using an IN criteria to load related items in bulk immediately after the lead query result is loaded.

Change-Id: If13713fba9b465865aef8fd50b5b6b977fe3ef7d Fixes: #3944

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