Add a peer abstraction.

[sholsapp]

We have various peer abstractions floating around the code base. Some uses are:

• In the consensus module.
• In the http module.

Both are used to identify the peer, but have different means of doing so. The consensus module makes use of the IP address as an integer. The http module makes use a struct sockaddr_in or IP address in string or integer form. The http module also needs a port, which is currently hard coded into the http module.

I think a peer needs things like the following:

• IP address
• Port
• ... maybe more

Additionally, implementing this abstraction would create a better home for various utils functions that convert IP addresses between the various forms we use.

[sholsapp]

A few more changes are needed to refactor the code:

• Request object needs to take a Peer object and provide a to_url method so that we can hide references to peers from Client object (currently, asks for peer ip and hardwired port to build URL that gets passed to RestClient).
• Server needs to create Peer objects from raw requests on the wire (maybe we need a constructor that takes a sockaddr_t)?

[rverdon]

I am looking at using the peer abstraction in the transport layer. To do that I am going to need a way to get at the IP and port. Perhaps it would be a good idea to add a _get_sockaddrin function.

It would just look something like:

struct sockaddr_in ip_addr;
memset(std::addressof(ip_addr), 0, sizeof(ip_addr));
ip_addr.sin_family = AF_INET;
ip_addr.sin_port = htons(port_integer);
uint32_t ipv4_addr = internet_address_integer & 0xFFFFFFFF;
ip_addr.sin_addr.s_addr = htonl(ipv4_addr);
return ip_addr;