multi: move payment state handling into MPPayment

This commit moves the struct `paymentState` used in `routing` into
`channeldb` and replaces it with `MPPaymentState`. In the following
commit we'd see the benefit, that we don't need to pass variables back
and forth between the two packages. More importantly, this state is put
closer to its origin, and is strictly updated whenever a payment is read
from disk. This approach is less error-prone comparing to the previous
one, which both the `payment` and `paymentState` need to be updated at
the same time to make sure the data stay consistant in a parallel
environment.

routing/mock_test.go

@@ -518,6 +518,11 @@ func (m *mockControlTowerOld) fetchPayment(phash lntypes.Hash) (
 
 	// Return a copy of the current attempts.
 	mp.HTLCs = append(mp.HTLCs, p.attempts...)
+
+	if err := mp.SetState(); err != nil {
+		return nil, err
+	}
+
 	return mp, nil
 }