Incorrect financial records are created for partially paid multiple memberships via Membership backofice form

Steps to replicate:
1. Create a membership price set with two membership types price option that belong to different organization
2. Then register for the two membership type A ($50) and B ($20)
3. The total amount is freezed to $70 as the sum of membership fee of A and B. Then change the status to 'Partially Paid'
4. After submit. Pay the owed amount via 'Record Payment' form.

As on 'Record Payment' form, the owed amount is $140 (2x70$) which is incorrect. This is because the corresponding two membership lineitems and financial records are created with total amount $70 each. Its because of the underlying code is written in such a way that for each membership new financial items are recorded that result into N times of owed amount for N memberships, unlike participant and contribution.

This was bug was encountered while fixing https://issues.civicrm.org/jira/browse/CRM-20569