Try to improve server performance

[HugoGresse]

• switch to a single units objects
• don't do anything if units are not updated
• only send changes units and towns + still use jsonDiffpatch for the moment

Changes:

• IA is now updated less often (less powerful)
• IA random useage is now runned only after it was last run
• IA start is slightly delayed on game start
• country with one town are no longer assigned to players at game start

Possible updates later on :

• use Map/Set for units?
• remvoe jsonPathDiff

[github-actions[bot]]

Visit the preview URL for this PR (updated for commit 85e4cdd):

https://bataille-c1e50--pr2-global-server-refact-etqm3q41.web.app

_{(expires Sun, 19 Sep 2021 16:14:48 GMT)}

_{🔥 via Firebase Hosting GitHub Action 🌎}