Do per-process worker heartbeats

[andytinycat]

It would be better if each child worker outputted its own heartbeat, a single heartbeat key won't be sufficient if there are multiple workers running. @jonp you had code for doing this. Will you merge it in? I think this feature is essential.

[jonp]

Oh my. For some reason I thought it was already in there. I'll add it in the next couple of days.

[jonp]

I've created a pull request: #7