Fine-tune socket configuration

[wenkokke]

We should probably set SO_KEEPALIVE. The fact that we haven't might explain the occasional resource vanished errors.

We should probably set SO_LINGER. This may help transmit messages that linger on shutdown.

While it isn't super necessary, given that we don't expect high-bandwidth control messages, we should probably explicitly set SO_RCVBUF.

We're currently simply closing the file descriptor associated with the socket, but we should probably use shutdown. If we don't using SO_LINGER won't have any effect.

[wenkokke]

Fixed by #38.

The new implementation sets SO_LINGER and SO_RCVBUF.