Support parallel execution of players

[mosser]

The arena now instantiate a new player for each engine.run() method call. As a consequence, it should be possible to use the parallel capabilities of Scala to support a parallel execution of each players.

However, the code should be analyzed to assess that no shared memory is used (initially it was the same Explorer instance that was running). Logs should also be adapted as for now they assume that the code is sequentially executed.