dgram: generalized send queue to handle close

Checklist

• make -j8 test (UNIX), or vcbuild test nosign (Windows) passes
• tests and/or benchmarks are included
• commit message follows commit guidelines

Affected core subsystem(s)

dgram

Description of change

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.

Backport of https://github.com/nodejs/node/pull/7066

cc @TheAlphaNerd