change mapper postfetch on save_obj() to use deferred column loading

[Mike Bayer]

possibly with a mapper option...but deferred loading should be the default. set up all the "default-bearing" columns in a common deferred group, then let it roll. we also might want to improve deferred's ability to name arbitrary "groups" for this.

[Mike Bayer]

i want this in the release since i want a cleanup of the deferred column loader to be more generic and reusable. once thats done this ticket is trivial.

[Mike Bayer]

2f8ebfef1d096dd5e012ca67b775cfde8778ba13

[Mike Bayer]

Removing milestone: 0.4.0 (automated comment)