Fix TLS socket read stall immediately after handshake

[mpilquist]

I ran in to an issue with http4s ember servers where JRE HttpsURLConnection clients would stall occasionally. The TLS handshake would complete but ember server would timeout while reading the request body, eventually resulting in a 500 response indicating a request timeout.

The issue was caused by unwrapped data remaining in the unwrapBuffer after the handshake completed. As a result, the request to read from the TLS socket would result in an underlying read on the raw socket, but there's no further data available, and hence the read would timeout.

I tried reproducing in a test in fs2-io but due to various timing and buffer sizing sensitivities, I was unsuccessful. I have confirmed a fix using this http4s test though: https://gist.github.com/mpilquist/82726e0f8ff740defca6a235bf3efd41