odb: add some documentation to the foreach() test

Add a struct for network callbacks

Yes, we can get the repository from the owner of the object, but having
it marked explicitly makes the API more consistent.

Add support for push-urls

* Removed the #define for defaults
* Promoted progress structure to top-level API call
  argument

Refactor checkout into several more-sensible
entry points, which consolidates common options
into a single structure that may be passed around.

Fix/issue 787