Local API payment request with encryption generates key on every Encrypt

[mvdzwaan]

When using the TerminalApiLocalService.RequestEncryptedAsync the current library generates the needed encrypt keys for each request.

Looking at the EncryptionDerivedKeyGenerator.Generate method, this methods creates the same key on each call, while still performing a Rfc2898DeriveBytes with 4000 iterations.

Could we improve performance of all calls by using a static key which only gets created once ?

Performance is not much of an issue when starting a payment, but when using VirtualReceipts we would like this call to be as performant as it can be.