Re-added a per-chunk event limit on the number of times pistons, dispensers and droppers can fire per second. These limits were in place previously but were removed when we introduced per-island limits for hoppers and spawners; the per-chunk event limits have been added back to help reduce lag caused by pistons, dispensers and droppers When the per-chunk limit of piston extension and block dispense events per second is exceeded, further piston extensions or dispenser/dropper activations are cancelled for that chunk for a while and a puff of black smoke is displayed over the affected block UPDATE: Fixed a typo in /rules (thanks EItan for reminding me!)