Bench: Check for existing token before generating new token (#158)

[ao-david]

Broadcast.go now calls AuthToken which first checks if a token for the given user already exists.

• If the token exists, the call returns
• If the token does not exist, the call generates a new token and saves it using GenerateToken.

This will prevent subsequent token generations of tokens which will not have refresh tokens (as only the first token will come with a refresh token).