[FEATURE REQUEST] Add eTag support for transaction-like functionality using the REST API

[WolfgangSenff]

Is your feature request related to a problem? Please describe. Currently, with the realtime database, we do not support actual transactions, even though Firebase itself does. We do not support them because the REST API's approach to transactions is convoluted, at best.

Describe the solution you'd like I'd like to utilize the built-in support they have for eTags so that we can guarantee that if an item is modified at the same time another is, the modification does not cause conflicts.

Describe alternatives you've considered None, really - this is nearly impossible to handle perfectly with alternatives.

Additional context https://firebase.googleblog.com/2017/07/introducing-conditional-rest-requests.html