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.
We've already set foRunner.canExit to true if pw == nil in getWrapperAndRun, and SandboxFilters don't support the Restarting interface, so I think we can get rid of this special case check here.