Use case:
- 2 invoices in foreign currencies, made at different times (and different exchange rates)
- 2 payments (same characteristics as invoices)
- allocate the 1st payment to inv A
- allocate the 2nd payment to inv A and then B so that A is fully paid, but B is not
- try to make the final payment on B to fully reconcile everything

Results (all in the payable account)
- let CB be the credit line of B
- let D2 be the debit line of 2nd payment
- let D3 be the debit line of the 3rd and final payment

Before this commit, the exchange difference computation dailed because:
- There were 2 amls that had an amount_residual and needed to be reconciled (CB and D2)
- Also, there was a structural flaw in the code that did not allow for multiple lines to have a residual

After this commit, we still make 1 move to log the exchange difference,
but we make exchange difference lines for each line that need to be reconciled: CB and D2

OPW 751710
Commit in master c36f70f6726e0c7ccca1002f78e4d402e27cae3d (2017-07-20)

Closes #18347

[FIX] account: testing partial and full reconciliation