Fix commit b7731f39 that inversed order of timing functions

Fix commit b7731f39 that reverse order between distributed synchronization and timer start/stop.

The timer must be started once all the processes have been synchronized and stopped once all processes are done with all the computation, so again, after processes synchronization.

@fpruvost, @agullo, @sthibault: please review I didn't do any mistake, and I'll merge it quickly.