Change UdpMetricSink to use UdpSocket::connect method?

[56quarters]

Potentially have the sink call UdpSocket::connect to set the address that will be written too.

Pros:

• We aren't doing the address parsing ourself
• Our code becomes cleaner
• Remove address from the struct

Cons:

• We're modifying the socket we're passed (maybe this isn't an issue since we take ownership of it)
• Cadence now won't compile with anything lower than Rust 1.9 (maybe also not an issue since we are planning on calling .set_nonblocking by default per Change from_udp_host to create UDP socket in non-blocking mode by default #14 which is also Rust 1.9+)

[56quarters]

This isn't going to be implemented since it ends up changing behavior when writing to non-existent servers.

Example using UdpSocket::send_to that doesn't result in any write errors:

https://play.rust-lang.org/?gist=3c41ee4e64d5174a46f96c03fb2a8bd5&version=nightly&backtrace=0

use std::net::UdpSocket;

fn main() {
    let socket = UdpSocket::bind("0.0.0.0:0").unwrap();
    socket.send_to("foo:5|c".as_bytes(), "127.0.0.1:8125").unwrap();
    println!("{:?}", socket.take_error());
}

Example using UdpSocket::connect and UdpSocket::send that results in an error:

https://play.rust-lang.org/?gist=7abe83a3abba7fc7c167395128cdd3d3&version=nightly&backtrace=0

use std::net::UdpSocket;

fn main() {
    let socket = UdpSocket::bind("0.0.0.0:0").unwrap();
    socket.connect("127.0.0.1:8125").unwrap();
    socket.send("foo:5|c".as_bytes()).unwrap();
    println!("{:?}", socket.take_error());
}

I don't know what to make of this. However, it will definitely result in a behavior change for the client for no good reason that I don't fully understand. It also seems counter to the idea of UDP: If I'm sending a UDP datagram I shouldn't care if the other side is up, down, or on fire.