Backing Map Implementation and Retry for Incremental State

[leerobert]

I think examples should be at the top level and separated in the normal directory structure we have now: example/ can contain the basic example topology example/incremental can contain the incremental topologies

That way we can use the normal maven directory structure for junit tests on the backing map and state and incremental map and state.

[boneill42]

Agreed. I'll move the simple and incremental examples.

[leerobert]

Ok I plan on working on setting up the backing map for incremental columns and creating the word count example for that today. It shouldn't be too much work. Do you want to take the lead on junit tests and better error checking for the backing maps?