Immediately kill workers to prevent straglers

[spalger]

Fixes #4737

Each Worker currently listens for the process "exit" event and attempts to kill it's child process to prevent leaving stragler processes. This was implemented by calling fork.kill(), which I assumed to work like process.kill(), but unfortunately it is async and doesn't send SIGTERM to the process in the same tick (a requirement of process "exit" listeners).

This fixes the implementation by switching to fork.process.exit(), which synchronously sends SIGTERM.

To replication steps from #4737:

1. delete your optimize/bundles directory

2. create an empty file at optimize/bundles/webpack.records

3. in a console tab, run this script to repeatedly log the iojs process list

   while [ 1 ]; do ps | grep iojs; sleep 1; clear; done

4. in another console tab run the kibana server in dev mode

   ./bin/kibana --dev

5. observe the server logging optimizer crashed with status code 1 and the process list growing by one each time you do _#_4

6. Finally killall iojs

[jbudz]

LGTM