Stale socket FD usage when using libevent + SSL.

jgehrcke reported seeing garbage data being sent on sockets when hitting the master or agents with a lot of HTTP requests.

I was able to reproduce this locally by running HTTP requests while hammering the code with HTTPS requests.

Looking at the code, it appears that accepted SSL sockets may be used after they are closed (see here).

This code may call SSL_shutdown after Socket::~Impl closes the fd. This means that if the fd is re-used, we may accidentally write a TLS Alert message to the re-used fd.