[Qt] Payment request expiration bug fix (re-done)

- this is based on #4122 (which can be closed)

Currently a payment request is only checked for expiration upon receipt.
It should be checked again immediately before sending coins to prevent
the user from paying to an expired invoice which would then require a
customer service interaction.

- add static verifyExpired() function to PaymentServer to be able to use
  the same validation code in GUI and unit-testing code
- extend unit tests to use that function and also add an unit test which
  overflows, because payment requests allow expires as uint64, whereas we
  use int64_t for verification of expired payment requests

src/qt/walletmodel.h

@@ -40,7 +40,7 @@ public:
     explicit SendCoinsRecipient(const QString &addr, const QString &label, const CAmount& amount, const QString &message):
         address(addr), label(label), amount(amount), message(message), nVersion(SendCoinsRecipient::CURRENT_VERSION) {}
 
-    // If from an insecure payment request, this is used for storing
+    // If from an unauthenticated payment request, this is used for storing
     // the addresses, e.g. address-A<br />address-B<br />address-C.
     // Info: As we don't need to process addresses in here when using
     // payment requests, we can abuse it for displaying an address list.
@@ -111,7 +111,8 @@ public:
         DuplicateAddress,
         TransactionCreationFailed, // Error returned when wallet is still locked
         TransactionCommitFailed,
-        InsaneFee
+        InsaneFee,
+        PaymentRequestExpired
     };
 
     enum EncryptionStatus