benchmark: cleanup child_process IPC benchmark

Squashed from:
- child_process: fix IPC bench to obey send() ret val
- child_process: fix IPC benchmark message has two more bytes
- child_process: use setImmediate for IPC bench

PR-URL: https://github.com/nodejs/node/pull/10557


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Anna Henningsen <anna@addaleax.net>