Slightly tweak algorithm to automatically set the number of instances: Instead of...

Slightly tweak algorithm to automatically set the number of instances: Instead of restricting the number of instances to 4, we now use '(cpu_cores / 2) + 2' instances for more than 4 CPU cores. For at most 4 CPU cores, we still use 'cpu_cores' instances. This way we can use more than 4 instances on CPU's with more than 4 cores, but the number of instances won't grow linearly with the number of CPU cores - should help a bit to avoid HDD thrashing.