Renamed FedoraDatastreams to generic FedoraBatch

- renamed fcr:datastreams to fcr:batch
- updated POST .../fcr:batch to accept batch modification requests for:
     - creating nodes with properties
     - changing node properties
     - adding or updating binary content
     - deleting nodes

   and allow the modifications to happen anywhere (relative to the given
   node)
- renamed dsid query parameters to child (as they can be used against
  any node, not just fedora:datastreams)

Fixes https://www.pivotaltracker.com/story/show/60876990