When a message doesn't have enough credits to get sent, the sender blocks in FC.down(), sending a credit request every max_block_time ms and will be able to unblock and send the message only when new credits arrive.

To prevent indefinite blocking (e.g. when the OOB thread pool is exhausted and new credits cannot be processed), add a property max_wait_time_per_message, which defines the max number of milliseconds a message will ever be blocked in FC, before it is sent. This is regardless of the number of credits left.