Fix _AdjustCandidatePool

Currently the ConfigWriter.MaintainCandidatePool returns node names, and
_AdjustCandidatePool uses them as such, but then it passes these to
context.ReaddNode which in turn passes them to jqueue.JobQueue.AddNode which
uses them as objects.Node instances.

Since this is currently the only usage, we change return type from
ConfigWriter.MaintainCandidatePool to be objects and adjust the logging of
their names, so that the auto-adjusement works.

Reviewed-by: ultrotter