Add try/catch/rethrow for the FDBStoreDataKeyValueIterator to manage a
past_version error that had escaped notice.
Move the exception check into FDBIterationHelper.
Clean up imports.
merge tjoneslo: Add try/catch/rethrow for the FDBStoreDataKeyValueIterator

Merge pull request #1258 from tjoneslo/sql-layer/Catch-FDB-exception-and-rethrow
https://github.com/FoundationDB/sql-layer/pull/1258