Addition of utilities for time elapsing measurement 

## Context 
We needed to control evolution of time in tests.

## Description
- Move of the `Chronometer` class from `codjo-mad-server` to `codjo-util`;
- Creation of the `TimeSource` interface to replace a call to `System.currentTimeMillis()` by a call to `TimeSource.getTime()` :
   * creation of the `DefaultTimeSource` class to represent the real time;
   * creation of the `MockTimeSource` class to simulate time in tests.