Check the service is available before record success

[lukaskupr]

Solve the situation when a request that was sent before the trip may immediately overwrite the success value

[ackintosh]

Thank you for your pull request.

I'm not sure this change makes sense because:

1. As shown at the Usage, users call isAvailable prior to calling success/failure.

2. It seems impossible to completely avoid the overwrite you mentioned, because the most key-value store does not support ACID transaction. 🤔