- speedup update of a project working copy

The new code tries to reduce the number of http requests for unchanged
packages (that is, packages that do not need an update).
In the worst case (the project wc's packages are unchanged and contain
services, for example), the new code needs #http requests of the old code + 1
http requests. But we can improve, once the "noservice" patch
(see http://lists.opensuse.org/opensuse-buildservice/2014-06/msg00067.html)
is merged.
Note: the semantics of the update process itself did not change.