Speed up process switching

[marcinslusarz]

We may not get real multi-process support (#183) for quite some time, but we can make process switching much faster (because currently it's extremely slow). Instead of opening the pool before each syscall and closing it after we can keep the pool open and close it only on blocking syscalls (like wait, read from a pipe, flock, etc). It's much more fragile than normal process switching, so it should be optional.