RTDB write size limit error

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### [REQUIRED] Step 1: Describe your environment

* Xcode version: 11.6
* Firebase SDK version: 6.28.1
* Firebase Component: Database
* Component version: 6.28.1
* Installation method: `Carthage`

### [REQUIRED] Step 2: Describe the problem

#### Steps to reproduce:

If you call `setValue` on a `DatabaseReference` with a value that is larger than the allowed write limit of 16mb (my case was 19mb), the supplied completion handler is never called. Furthermore any subsequent updates from the client are never synced to the server.

I have not created a project to demostrate the issue as I think that it is fairly easy to reproduce.
If you wish, I could create a reproduction.

#### Relevant Code:

```
let value =
ref.setValue(value, withCompletionBlock: { error, _ in
print("Never reached")
})

// sleep for a while
otherRef.setValue(smallValue)

// smallValue never reaches the server

```