Event order + Fixes

[Matt-Esch]

Worker state change events need to be hooked up before the process is started. This is achieved by initializing the process on next tick.

However, this breaks the assumption that worker.process exists immediately after initialization. To work around this, we re-emit the process message on the worker and subscribe to worker.on('message', ...) instead.

Also fixes a minor edge case where workers do not restart if the restart delay is 0.

[kriskowal]

Looks good to me.

[claassistantio]

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