use lumberjack for log rolling

This is very similar to the last PR, except I didn't change out the upstart config stuff... since the output redirect doesn't hurt anything, and we need the rest of the code to properly set permissions and owner on the logfile.
The compiler doesn't care either way, it'll produce the exact same code.  In cases like this, where they're values that'll never get used anywhere else and where the values are just assigned to obviously-named fields, there seems to be no benefit to making them named constants.

const maxLogSize = 300
const maxBackups = 2

log := &lumberjack.Logger{
		Filename:   filename,
		MaxSize:    maxLogSize, // megabytes
		MaxBackups: maxBackups,
}

I don't think that's any better.