Eliminate principle nodes

[CJ-Wright]

Principle nodes are annoying, they require lots of knowledge about how a pipeline operates exactly. They are currently used to designate one or more nodes as nodes which signal to event stream nodes to issue start and stop documents.
It may be possible to inspect the graph in such a way as to not need to do this.

One could examine all the From nodes and then once one get a start doc have the To node issue a start doc. One difficulty here is that we may ingest docs from across headers, which would trigger multiple starts.

One would need to do something similar for stop docs.