Add a way to get the file descriptors for use with epoll

[i509VCB]

At the moment I don't see a way to take DuplexConn or the higher level RpcConn and get the file descriptor(s) that I can use in epoll for not blocking or needing to poll like crazy.

[cmaves]

AsRawFd is implemented for DuplexConn which gives the receiving RawFd. If your using RpcConn, you can use RpcConn::conn() to get the underlying DuplexConn first.