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.
It's "accidentally constant" because it's difficult to adjust in the reverse direction, which is what most clients use. It should be adjusted in the forward direction, but there's not all that much point. The best solution would likely be either to pass the extra length in the marker (very, very tricky if all the responses come from a single ledger) or to make the query string a bit more complex. This is a very ugly bit.