Update done tasks.
UPnP mapping done for now.
Add chaining constructors to simplify building processing chains.
Add logging in set_enabled() calls along the chain.
Don’t dequeue if there’s nothing.
Add disable call in packetizer dtor.

It’s not technically necessary, since packetizer is the last element
in the chain.
Add set_source() member function.
Accept received packet as an input.
Signal empty queue after clear.
Reserve 12 bytes for timestamp and sequence number in encoder/decoder.
Empty buffer if there’s nothing to encode.