Ending the game when player's or opponent's health reaches nil

[bzns]

• When an active player playing the game reaches 0 health points, the game instance is stopped and the final stats are returned to the client at its next /games/{gameId} GET request

Estimated time: 3h This task is part of user story #24

[Thahit]

no multiplayer

[Thahit]

now deletes game when leaving/losing https://github.com/sopra-fs21-group-10/td-server/commit/f5ae81890511763f636d900f6f0a554a44f86b64 (=test)