Think through the API, make it more consistent

[Kagami]

• var_int.value vs var_str.str vs var_list.list
• net_addr.addrs vs inv.inventory vs error.vector
• pubkey.needed vs msg.identities vs broadcast.subscriptions
• bootstrap/connect/listen API: it's better to always use objects instead of positional arguments. (E.g. {host: "1.2.3.4", port: 8444})
• connection pass 3 positional arguments (should it pass transport, {host, port} instead?)
• error message fatal field is nonintuitive (better to use something like type)
• version message version field is nonintuitive (better to use something like protoVersion)
• version message streamNumbers field is awkward
• High-level checkings (like connection to self detection) probably should lie in transport code
• Exceptions in transport callbacks propogated further: do we want to auto-catch and warn about them?
• Somehow make it clear that methods like tryDecode and validate don't throw exceptions