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.
Not sure it makes sense to remove this. An error in NewRetryHelper means that the one of the specified time durations couldn't be parsed, which means that there's a config error, and for every other config error we bail out and shut down.