Unsubscribe from 'token_received' events before re-subscribing

[l1b3r]

OAuthService::setupRefreshTimer is called multiple times from several places in the code, e.g. from the constructor of the service or when a storage is changed via OAuthService::setStorage.

The setupRefreshTimer is itself subscribing to OAuthService::events each time it's called, and the subscriptions get stacked creating at the very least a memory leak. In the worst case, this can also lead to undefined behavior and race conditions since the subscription handler of this.events... in setupRefreshTimer is calling setupExpirationTimers which in turn is also performing subscriptions.

This PR adds a resubscription procedure to setupRefreshTimer which will unsubscribe from OAuthService::events before subscribing to them again.