Just use raw close() for discard

[jhawthorn]

Follow up to #65 cc @casperisfine

Previously we were doing a dance with file descriptors in order to replace the connection's fd with one open to /dev/null. Instead we should be able to just close() the fd so long as we don't perform any writes on it.

[casperisfine]

so long as we don't perform any writes on it.

But that's actually the point. To make sure we won't flush any buffers etc.

Is this dance causing issues?

[jhawthorn]

In practice most of the time this probably works fine, but it adds some dangers and unnecessary complexity. open could fail because of fd exhaustion. Calling SSL_shutdown may push errors (which we should be cleaning up anyways). It all should work, I'd just like something similar and not to have an extra state to consider connections could be in (being connected to /dev/null)

I've opened #111 which simplifies this even further and combines the shutdown and discard callbacks.