implement Socket.setsockopt()

[fafhrd91]

we need this for some options, like TCP_NODELAY, TCP_CORK, SO_KEEPALIVE

[1st1]

NODELAY should be set by default btw.

[fafhrd91]

But if we start mixing it with CORK, then we need to turn it off?

@asvetlov added this cork and nodelay manipulation to aiohttp

[1st1]

Well, asyncio in Python 3.6 (and 3.5.3) sets NODELAY by default and so should we. It's really up to those who use advanced TCP settings to workaround that.

[1st1]

Moreover, I hope we can use (eventually) vectorized writes in tokio.Transport.write() and .writelines(), which should cover use cases for CORK.

[fafhrd91]

btw we need to add tests for Transport, uvloop doesn't contain tests for "writelines()" for example