Persist in-memory contract sessions

[LennardF1989]

Currently, if Peacock crashes for one reason or another, all in-memory contract sessions will be lost. While you can turn on the options flag to allow you to reconnect to Peacock without closing the game, you will have lost your progression on the contract.

We can probably come up with some solution to (occasionally) write contract sessions to the file-system like we do when making a save, and clean them up again if no saves are associated with it x amount of days after creation (to prevent Peacock from removing them immediately on restart).