Add eco mode

This MR uses the number of test per second to evaluate if the duniter is too fast (I set it to 300 test/s for now : it is one of my i7 core at 30% of CPU usage). At first the eco mode reduce number of cores and when there is only one core left it reduce CPU usage.

For now the configuration is not saved so if the node is restarted the config will comeback to the initial state.

Any remark is welcome !