child_process: emit IPC messages on next tick

Currently, if an IPC event handler throws an error, it can
cause the message to not be consumed, leading to messages piling
up. This commit causes IPC events to be emitted on the next tick,
allowing the channel's processing logic to move forward as
normal.

Fixes: https://github.com/nodejs/node/issues/6561
PR-URL: https://github.com/nodejs/node/pull/6909


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>