This commit refactors AMR streaming framework and add support for a mechanism
for streaming composite datasets in general. To try out amr streaming, run all
executables with "--enable-streaming" flag and open an AMR Flash or Enzo dataset
and render as "AMR Blocks" or "Volume" (currently, only works in builtin mode,
but will be supported in other modes soon).

vtkAMROutlineRepresentation is a example class that demonstrates how a
representation can leverage the streaming capabailities provided by
vtkPVRenderView.

vtkPVRenderView adds support to two new passes: REQUEST_STREAMING_UPDATE() and
REQUEST_PROCESS_STREAMED_PIECE().  REQUEST_STREAMING_UPDATE() is a view pass in
which the representations can update their data pipelines to request a certain
"piece" of the data. REQUEST_PROCESS_STREAMED_PIECE() is called on
representations to handle a "piece" just received by the process. This
separation makes it possible for representations to be agnostic of where the
data is processed and where it is rendered. It's the reposibility of the view
(vtkPVDataDeliveryManager in particular) to deliver the data to the
rendering nodes.

vtkPVDataDeliveryManager doesn't currently support data delivery for streamed
pieces, however. That's on the TODO list.