fix benign race and clean up handshake logic

[Stebalien]

• Split lazy server and lazy client types. They're very different and do very different things.
• Combine lazy client write with handshake. This allows us to negotiate and write all in one go.

I'm not happy with the server-side logic but couldn't find a nice, clean way to fix it so I left it as it was (mostly). Eventually, we'll have to switch to a new protocol anyways because this one isn't sound.

[Stebalien]

Note: there's one TODO. The test cases assume that reads with zero-length buffers will succeed instantly. This can hide errors and is wrong. Should I just fix the tests?

[coveralls]

Coverage decreased (-2.2%) to 77.262% when pulling ae3268d28241403197ee0dad40d2bd39f1163da9 on fix/race into b8f1996688ab586031517919b49b1967fca8d5d9 on master.

[coveralls]

Coverage decreased (-1.9%) to 77.506% when pulling ae3268d28241403197ee0dad40d2bd39f1163da9 on fix/race into b8f1996688ab586031517919b49b1967fca8d5d9 on master.

[Stebalien]

Feedback addressed.