dgram: generalized send queue to handle close

If the udp socket is not ready and we are accumulating
messages to send, it needs to delay closing the socket when
all messages are flushed.

Fixes: https://github.com/nodejs/node/issues/7061
PR-URL: https://github.com/nodejs/node/pull/7066


Reviewed-By: Anna Henningsen <anna@addaleax.net>