App crashes when using Storage from a Firestore completion handler

### [REQUIRED] Please fill in the following fields:

* Pre-built SDK from the [website](https://firebase.google.com/download/cpp) or open-source from this repo: **website**
* Firebase C++ SDK version: **7.1.1**
* Main Firebase Components in concern: **Firestore, Storage**
* Other Firebase Components in use: **Auth**
* Platform you are using the C++ SDK on: **Mac**
* Platform you are targeting: **iOS**

### [REQUIRED] Please describe the question here:

#### Briefly
Is it safe to call other firebase methods from a future `OnCompletion` handler? That is, start a second (or more) asynchronous task when the first completes?

#### In detail
This is potentially a regression, but could be a misuse of the API on our part. We've been using Firebase 6.15.1 successfully for a while and decided it was past time to upgrade to 7.1.1. We have a design where configuration is stored in Firestore, which tells the app which files are required from Cloud Storage. So for one call to our internal function we need a call to Firestore followed by a call to Storage. Simplified code would be something like:
```cpp
void getCorrectFile( const char* localPath, std::function callback )
{
pFirestore->Collection("configStuff").Get().OnCompletion(
[callback=std::move(callback)]( const ::firebase::Future& collectionFuture )
{
if( collectionFuture.error() != firebase::firestore::Error::kErrorOk )
{
callback( "An error occurred (real code maps errors to error_codes)" );
}
else
{
// ...code to work out correct file from the QuerySnapshot...
firebase::storage::StorageReference fileRef = pStorage->GetReference(firebasePath);

fileRef.GetFile(localPath).OnCompletion( [callback=std::move(callback),fileRef](const firebase::Future& getFileFuture)
{
if( getFileFuture.error() != 0 ) callback( getFileFuture.error_message() );
else callback( "No error (real code would create default constructed error_code)" );
} );
}
} );
}
```

As I say, this all worked fine in Firebase 6.15.1. After upgrading to 7.1.1 we now get various crashes[*]. Before investigating I thought it was worth checking we're not fundamentally abusing the thread model in the API, and it just happened to work previously.

[*] How it crashes is fairly inconsistent. It can be an assert from some mutex.h file, or an unhandled `*** -[NSFileManager createDirectoryAtURL:withIntermediateDirectories:attributes:error:]: URL is nil` exception. I've even seen it work once.