A notion of default node

Per CAY-943, Cayenne can merge multiple configurations into a single runtime. Now using that in a real system, I ran into a scenario that I have to define DataNode over and over again with the same name across multiple projects. It would be nice to have DataMap-only projects that would link to a "default" node.

Will add a "default node name" property to DataChannelDescriptor (so that DataChannelDescriptorMerger could set that). Since 99% of projects are single node, if "default node name" is null, that single node will be used as default (the merger probably can take care of that). DataDomain.lookupDataNode would route all unlinked maps to the default node.

Also DataDomain.lookupDataNode should probably throw on NULL.