Implement additional UDP features

[camshaft]

It would be useful to extend the current UDP model with the following features:

• Randomized packet corruption/truncation
• Randomized packet duplication/retransmission
• Randomized packet reordering - this can be accomplished by having some jitter assigned to each packet.
• Preferring new packets instead of old on full receive buffers - currently we drop new packets on full buffers but this isn't usually what network stacks do or what applications expect. - turns out this is exactly what stacks do - see Drop oldest udp message when buffer is full #128 (comment)
• Setting the MTU for a path and being able to drop and/or truncate packets larger than that value
• Simulate bufferbloat (i.e. latency increases by some function as the number of packets being buffered increases).

[mcches]

@camshaft was Preferring new packets instead of old on full receive buffers - currently we drop new packets on full buffers but this isn't usually what network stacks do or what applications expect. already done? If so, can we check it off?