cluster: support datagram sockets
dgram: avoid EventEmitter leak warning

When a datagram socket hasn't been bound yet, node will defer `send()`
operations until binding has completed. Before this patch a `listening`
listener would be installed every time `send` was called. This triggered
an EventEmitter leak warning when more than 10 packets were sent in a
tight loop. Therefore switch to using a single `listening` listener, and
use an array to enqueue outbound packets.
net: Avoid tickDepth warnings on small writes

Always defer the _write callback.  The optimization here was only
relevant in some oddball edge cases that we don't actually care about.

Our benchmarks confirm that just always deferring the Socket._write cb
is perfectly fine to do, and in some cases, even slightly more
performant.