Implement cb-depsolver

[schaefi]

The cb-depsolver service should be used to trigger build requests if the dependencies to build a package/image has changed. The information about dependencies is already present for built packages/images in the .solver.json file and can always be directly calculated via cb-ctl --build-dependencies-local --arch ... --dist ...

The dependency information is presented in a json like format of the form

{
    "image": "xsnow",
    "resolved-packages": {
        "adduser": {
            "arch": "all",
            "installsize_bytes": 869376,
            "source": "http://ftp.halifax.rwth-aachen.de/debian/dists/unstable/main/binary-amd64",
            "status": "added_by_dependency_solver",
            "version": "3.118"
        },
        "adwaita-icon-theme": {
            "arch": "all",
            "installsize_bytes": 27046912,
            "source": "http://ftp.halifax.rwth-aachen.de/debian/dists/unstable/main/binary-amd64",
            "status": "added_by_dependency_solver",
            "version": "41.0-1"
        },
        ...
}

The implementation of the cb-depsolver service should perform the following steps

• Calculate the solver result via the existing satsolver based dependency solver for each package/image
  basically what cb-ctl --build-dependencies-local --arch ... --dist ... does
• Compare the former result of a solver operation with the current one and provide a diff information if there are changes
• From the changes decide to create a build request for the package
• Periodically run this solver operation