SIG ingress pipeline

Incoming packets are processed in the following manner:

1. Disapatcher (singleton) object reads SIG frames from the network and passes them to an appropriate Worker based on the source IA, source host address and session ID.
2. Worker passes the frame to a ReassemblyList based on the epoch. Non-active epochs are purged in periodic manner.
3. ReassemblyList keeps a list of frames. It processes them in a lazy manner: It only parses the content once an entire IP packet can be assembled. The reason for this is that there may be holes in the frame sequence and in that case we want to drop the old frames. Which wouldn't be possible if the frames were processed immediately as they arrive.
4. Once a full packet is available, it is sent to the local network via the TUN device.