TransferQueueBundler: drop messages if queue is full

When a bundler (like TransferQueueBundler) has a full queue, the senders will block.
This can happen for example when the TCP send blocks due to a full send-window.

Retransmission requests and responses compound the problem when the TQB's queue is full.

Therefore, in addition to blocking, we should add an option to drop packets when the queue is full. Investigate whether we could use some priority based scheme, which drops non-important messages first.

Also investigate whether senders can selectively be unblocked when the queue is full, to either drop or add their messages. This could be done for example by removing elements from the queue, using the aformentioned filter.

Look at random early detection )[1]) for inspiration.

[1] https://en.wikipedia.org/wiki/Random_early_detection