Integrate `tlsfuzzer`

[webknjaz]

I suggest changing the description of this issue to "OpenSSL.SSL.WantReadError and WantWriteError", because both errors occur for similar reasons. (I encountered the one with WantWriteError, which is probably the more likely of the two because you're most likely to send big data than receive, but I'm only guessing.)

Thanks a lot @jpeckham, your workaround helped me.

If anyone can't reproduce the issue, this indicates that the problem might be OS-dependent.

And, by the way, pyca/pyopenssl#176 says that sendall() should not be used at all until fixed because the same exception can occur and will cause loss of state.

@webknjaz, I was originally using the BuiltinSSLAdapter, but the server then failed some tlsfuzzer tests (specifically I think it was test-fuzzed-ciphertext.py) and eventually stopped responding after these fuzzing tests or after a Qualys test. That's why I switched to pyOpenSSLAdapter, which works better in this regard but has the issue with these exceptions, which seems much easier to fix than the fuzzing failures. (Of course, if anyone happens to know why that happens or how to fix it, I'd welcome that information.)

Originally posted by @vashek in #245

I've been meaning to see if we can stick tlsfuzzer into our test suite but looks like I haven't documented the intent. Making this issue to have this explicit.

I started looking into it last year but it's not easy to integrate into our deps atm: tlsfuzzer/tlsfuzzer#903.