Socket leaks on async connect failure

[dholroyd]

I've had long-running applications run out of file descriptors and while debugging noticed that srt-rs will leak a socket if async connect fails.

I worked around that by implementing Drop for ConnectFuture in dholroyd@5e6adce

I can submit that as a PR if the approach is good. I had originally planned to implement Drop for SrtSocket, but that looked much more difficult due to lots of existing usage needing it to be Copy.

[nthieu173]

The ConnectFuture move out of scope and gets dropped once it returns a successful SrtAsyncStream. So this approach will close a socket while it's still in use. Instead, I think closing it once there is an error in poll() is a better approach.