Persist retrieval market counter to disk

[hannahhoward]

Goals

Make retrieval deal id incrementing survive restarts so they can serve as unique deal ids for a single retrieval client (fix #66 )

Implementation

• Create a simple stored counter interface that saves an incrementing counter on top of a data store
• Use it in retrieval market

For Discussion

• The integration test recreates all the test data on each retrieval and therefore does not test the counter -- should it?
• This should go in after Use go-statemachine + FSMs in retrieval market #124 cause it's on top of it (I currently set that branch as the base)

[shannonwells]

looks good to me. I'm not sure whether explicitly testing the counter in the integration test is necessary; my kneejerk reaction was "of course you should test it in the integration test", but it looks like the unit tests are good enough.