id for setTimeout and setInterval are prone to errors

[luiza-cicone]

Hello,

We are using the verification client setTimeout method to compute a custom viewability.
As it is using an internal generation of id's (starting at 0) it is prone to errors as it is using a falsy value and so we need to explicitly check against undefined
if (timeoutId !== undefined) { ... } instead of if(timeoutId) { ... }

I can suggest a PR where we change the id returned to start at 1. WDYT?
https://github.com/InteractiveAdvertisingBureau/Open-Measurement-JSClients/blob/master/src/verification-client/verification-client.js#L381

Thanks!