Introduce TLSStream

[sergiu128]

To support TLS connections, we currently wrap the struct returned from tls.Dial into an AsyncAdapter. This is not ideal because:

• TLS might need to also read on a write and vice versa depending on how renegotiation is setup (off by default). The async adapter might block if we must read on a write, since it only checks if a write is possible.
• Calling asyncAdapter.Close() closes the TLS file descriptor, bypassing the closing handshake, which is incorrect. As such, users must invoked tlsConn.Close() and skip calling close on the async adapter, which is cumbersome.