lazily create worker threads

[mndrix]

Jolog currently spawns CpuCount*2 threads on startup. For many programs, we don't need that many threads so starting them is wasteful. Instead, start with one worker thread. Each time the manager thread prepares to block (because it's outpaced the workers), start a new thread. At most CpuCount*2 workers should ever run.

There are many more optimizations we could make to tune worker count based on CPU usage and blocking IO usage, but this small adjustment is easy and should help quite a bit.