FC: messages > max_credits will block forever

FC.down checks for the message size. If that size is greater than the credits left, we block until we get more credits from the receiver(s). However, if the message size is greater than max_credits, we will never have more than max_credits available to send the message, so we'd block forever !

To prevent running into this, we need to check (at startup time) in FC that

• a fragmentation protocol is available (somewhere above the FC protocol)
• that the max fragmentation size of the fragmentation protocol is smaller than max_credits

We should also log a (fatal) error message if we receive a message whose size is greater than max_credits (this should not happen anymore though after the proposed change above)