Handle removing of projections and federated nodes

Currently there is no special handling for 2 important "remove cases":

1) the node.remove() call on federated nodes: this will delete the federated node and with it all its projections, but the system area where we store the projection mappings would be left unchanged and therefore in a corrupted state. Also, we need to make sure that calling node.remove() on a federated node or an ancestor of that node does not cause a write connector to remove all external content.

2) removing a projection from a federated node: currently we don't have this possibility (technically). One option is to enhance the FederationManager and add a removeProjection method which only removes an external segment from a federated node, leaving the rest of it unchanged.