[experimental] Suspend/restart transaction confirmation when network goes offline

[steveluscher]

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The job here is to augment every transaction confirmation algorithm in the new web3.js to detect offline events in browsers. This means to suspend transaction confirmation when the offline event fires on window and sensibly resume it when the online event fires.

More information on this browser API: https://developer.mozilla.org/en-US/docs/Web/API/Navigator/onLine