Validate and parse incoming gossip messages

[jannikluhn]

• parse them and the included data types
• convert them to a struct (one per message type)
• the struct should conform to an interface (c.f. gossip message interface already defined in shmsg)
• reject messages that are not parsable or have easily detectable invalid values

The libp2p validation interface should be able to do both parsing and validation at the same time. If not, revise.