Update ExternalProcessed flag functions; if Timestamp < 0 WHERE clause should check if Null

"UpdateProcessedExternal" functions in the Business Layer pass in the pk and timestamp of the record to update. The purpose of timestamp is to stop the record being updated where it may have been changed by another user.

A change has been made to make this more flexible.

If you pass in a negative Timestamp, instead of testing that the timestamp field matches, the function will test that the current "ProcessedExternal" flag is Null.

Changes have been made to:

AccountFunctions; DisbursementUpdateProcessedExternal and AccountingLedgerUpdateProcessedExternal
AccountSecuritisationFunctions
AccountTransactionFunctions
BankTransactionFunctions
MailboxFunctions