happy eyeballs

[njsmith]

Whatever our create_connection looks like (#8), it'd be neat if it automatically did happy eyeballs :-). This should be pretty trivial (though it'd help if #7 was sorted), basically something like:

async def create_connection(host, port):
    first_family = None
    results = await getaddrinfo(...)
    # pick the first two results with different AF_* settings
    first_res = results[0]
    second_res = [res for res in results if res[0] != first_res[0]][0]
    async def connect1():
        return await connect_to(first_res)
    async def connect2():
        await trio.sleep(0.300)
        return await connect_to(second_res)
    # 'race' runs several tasks, returns the one that finishes first, and cancels the others
    return await race(connect1, connect2)

(+ the mandatory caching the RFC 6555 specifies)

[njsmith]

Fixed by #145