Civiaccounts inconsistent with spec when adding partial event payment

New Description from Joe: Refactor the work done by core team that implemented partial event payment to remove problems associated with not following the Data Flow specs in http://wiki.civicrm.org/confluence/display/CRM/CiviAccounts+Data+Flow#CiviAccountsDataFlow-Updatecontribution.

In particular: NEVER change values in existing records in the financial_item and financial_trxn table since they provide the history that is needed for auditors. Instead, additional rows are added to these tables so that the bookkeeping accounts will correctly reflect the changes that have been made from an accounting perspective.

It is not clear whether it is feasible to code the recreation of financial records during upgrade.

Original description from Jake: According to the spec at http://wiki.civicrm.org/confluence/display/CRM/CiviAccounts+Data+Flow#CiviAccountsDataFlow-Pay-latercontributionreceived%28updatePendingPayLaterstatustocompleted%29 , when adding a partial pay-later payment (or any subsequent partial payment) to a participant/contribution record, the entity_financial_trxn table needs to link the trxn to the contribution and to the financial_item. Right now, only the contribution link gets created.

Note that this bug affects only partial payments; full payments create the correct entries.

Tagged as MIH b/c I believe CiviAccounts was originally a MIH; tagged User and Admin doc b/c a fix might require a user to be aware of inconsistencies.