-Add: Allow fixed-length-packets to be sized.

This allows us to use a Packet to identify the start header of each
packet.
-Change: remove LRU cache as this might interfere with late registering.
-Change: remove superfluous print statements.
-Change: get_packets_dict for direct(er) access to the protocol's packets.

get_packets now uses get_packets_dict internally.
-Add: Beginning of the PacketSockets.