account_tx resume bugfix

This is a fix for RIPD-314. It's not a perfect fix, but it's a good temporary fix until the existing code can be improved.

This fix works by ensuring we get enough transactions to ensure we can find the marker transaction, even if the limit is small. A better fix would be to either encode the necessary number of transactions in the marker or to modify the query to always start from the marker transaction.
Is this a constant?  At the very least, it should be static const.  Perhaps it should be brought up to a separate header file, like Tuning.h?  We have some many magic numbers.  What does EXTRA_LENGTH really do, anyway?  Perhaps a clearer name?