Filters/Outputs can optionally exit without shutting down heka

This is the first pass, I'd like to get some review on it, and I've tested it
with a few different plugins, both filters and outputs, and it seems to work.

I'd appreciate it if someone else could put it through its paces to make sure
I haven't overlooked anything.
Yes, I specifically wanted to set the err return value to this. The foRunner code logs errors returned by Run(), and by returning a non nil error in the SandboxFilter, we can determine if it's sent a termination message already.