TP: ignore DONT_BUNDLE flag for sending of messages

Currently, messages tagged with DONT_BUNDLE are sent separately, not as a message bundle through a bundler. On the receiver, they are passed up as single messages, ie. on a separate thread.

Sending a lot of single (possibly small) messages is inefficient; it would be better to bundle all message on the sender side. This requires a bundler which sends single messages immediately when no other messages are available to be sent.

SOLUTION (sender):

• If the bundler is not "old", ignore the DONT_BUNDLE flag and send the message through the bundler
• Else, same as now; send the message immediately as single message

SOLUTION (receiver):

• When reading the message list and creating the 4 batches (regular, oob, oob+internal, internal), if a message has DONT_BUNDLE set, pass it as single message to the corresponding thread pool and don't add it to the batch

This has the advantage that we have a more efficient way of sending messages (as message bundles), yet the behavior (and performance) at the receiver is the same as now