Add flag HIGH_PRIO (or PRIO), which

• on sending: bypasses the bundler when sending a message
• reception: bypasses the thread pool for regular messages
• reception: for OOB messages -> use the thread pool. If full, use the current thread to pass the message up

This is beneficial for certain types of messages, which don't block and don't send a response, e.g. credit responses:

• The receiver never drops a credit response (might only block on the transport (e.g. TCP write) (UDP might still drop the message on a full send-buffer, but the message will get retransmitted).
• The sender gets the credit response even if the thread pool is full