Create IO threads dynamically

[RavenX8]

In network_thread_pool.h we create a fixed number of threads to handle I/O operations. It would be better to spawn threads until the CPU usage hits a threshold. This would ensure that we are running at the maximum performance for the server.

Ref on how to get cpu usage: http://stackoverflow.com/questions/63166/how-to-determine-cpu-and-memory-consumption-from-inside-a-process