Add iteration methods to IterationHelper, remove direct usages of Exchange
Use IterationHelper in cursors. Exception cleanup.
Rename
More id into StoreAdapter and use base class in row pool.
More exception, import cleanup
Javadoc interface
merge nwilliams: Refactor IndexCursor implementations to work in terms of the IterationHelper interface.

https://code.launchpad.net/~nwilliams/akiban-server/abstract-iteration-helper/+merge/172176