Realtime Database Async update hangs on error

### [REQUIRED] Step 2: Describe your environment

* Operating System version: Docker openjdk:8 running on Google Cloud Run
* Firebase SDK version: 8.0.1
* Library version: 8.0.1
* Firebase Product: RT Database (auth, database, storage, etc)

### [REQUIRED] Step 3: Describe the problem

I made an update request with a completionListener with a badly sanitized key that had " and :. While I handled errors, this caused the request to end with an Internal error code. I forced the database call to timeout and received an error saying that my input was malformed.

Expectations: if the call to the database is malformed, it should just quit immediately instead of hang.

#### Steps to reproduce:
1. Call the updateChildren function with a malformed key
2. Force the callback to terminate (I used 5 seconds)
3. You should see an error s.a Firebase Database error: Invalid token in path