Allows to only import nodes without any relationships

For this occasion InputNode ids are allowed to be null, meaning that
such nodes are anonymous and cannot be found when creating relationships
later on in the import. Also the :ID header entry in CSV input is now
optional and if the :ID header entry is present nodes can leave out value
in that column to have such nodes be anonymous.
Ability to keep ids as properties on nodes

If the :ID header is named, e.g. myId:ID in a CSV input, then each node
having an id in that field will also have that value stored as a property
with the name of the header entry, in this case "myId".
Merge pull request #3544 from tinwelint/2.2-bip-flexible-id-management

More flexible handling of node ids during import