Tweak CELL_STATS event based on comments by nickm.
- Move cell_command_to_string from control.c to command.c. - Use accessor for global_circuitlist instead of extern. - Add a struct for cell statistics by command instead of six arrays. - Split up control_event_circuit_cell_stats by using two helper functions. - Add TestingEnableCellStatsEvent option. - Prepare functions for testing. - Rename a few variables and document a few things better.