Race condition when starting multiple games at the same time

[Leonabcd123]

The last game's final balance overrides the first game's resulting balance.

Steps to reproduce:

1. Start banana-rps game
2. Start banana-flip game
3. Finish banana-rps game
4. Notice final balance is the one at the end of banana-rps, and it does not consider the result from banana-flip. This means that if you run:

/banana-rps 100
/banana-flip heads 100

with starting balance being 100, and you win in banana-flip but lose in banana-rps, your final balance will be 0.

Expected Behavior

The user's balance should decrease when the game starts, so the example shown before would look like this:

// Balance: 100

/banana-rps 100

// Balance: 0

/banana-flip heads 100

// Error: You don't have enough bananas.