Investigate the use of parallelism for the game server

[Longwelwind]

At the moment, the game server is a single (mono-threaded) JS process that handles all the games loaded. While this is a good first implementation, and it never posed any issue for Swords & Ravens which sustained some load, it may become problematic if a game is particularly popular.

Some possible path to improve that:

• https://nodejs.org/api/cluster.html
• https://nodejs.org/en/knowledge/getting-started/the-process-module/