Sketch of implementation.
PushVolume implements IStateChange.
Create utility function that constructs the standard INode provider that
volume managers use to communicate with each other.
PushVolume implementation.
Change orchestration algorithm to do an initial push.
Document the new feature.
Lint fix.
Fix document structure while I'm at it.
Merge remote-tracking branch 'refs/remotes/origin/master' into two-phase-handoff-373
Rename document to omit references to orchestration.
Remove references to orchestration in the prose.
Address review comments: Reduce the excess verbiage and explain things in more uncomplex terms.
Address review comment: Reference explanations of jargon.
Address review comment: Docstring fixes/improvements.
Address review comment: Better explanation.
Address review comment: Better explanation.
Fix reference to renamed document.
Merge remote-tracking branch 'refs/remotes/origin/master' into two-phase-handoff-373

Conflicts:
	docs/advanced/whatsnew.rst
Merge pull request #696 from ClusterHQ/two-phase-handoff-373

Two phase handoff.

Fixes #373.
Using a filter causes a PicklingError on my mac. Test if this is the case on Buildbot.