Add try/wrap/throw to the FDBStoreDataIterator implementations to ensure
exceptions are processed correctly. 

Clean up a few imports as well.
Add more processing to the TransactionState, refactoring in a few calls
to get<stuff> which wanted only to wait for the results. Pull these into
the TransactionState and add try/wrap/throw
fix all the calls to Future.get() with try/wrap/throw by having them all
go back through the base PendingCheck<> class.