[Bug] Nested `Future::OnCompletion` appears to deadlock

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

* Pre-built Firebase C++ SDK version: 12.1.0
* Problematic Firebase Component: Future/Functions
* Platform you are using the C++ SDK on: Mac
* Platform you are targeting: Desktop

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

As the title says, nested `Future::OnCompletion` appears to deadlock.

I have created a sample project reproducing this problem at:
https://github.com/DistroAV/firebase-functions-future-async-nested

I have a very simple firebase function running in an emulator (the same problem happens when published to project too):
```JavaScript
exports.foo = onCall((data, context) => {
return 200;
});
```

I then have C++ code to call this:
```C++
firebase::Future CallAsync(const char *name, const firebase::Variant &data,
std::function
completion_callback)
{
cout << "tid=" << pthread_self() << " +Cloud::Call(`" << name << "`, ...)" << endl;
firebase::Future future;
if (functions_) {
auto httpcallref = functions_->GetHttpsCallable(name);
cout << "tid=" << pthread_self() << " Call: `" << name << "` +Call(...)" << endl;
future = httpcallref.Call(data);
cout << "tid=" << pthread_self() << " Call: `" << name << "` -Call(...)" << endl;
future.OnCompletion(
[completion_callback](
const firebase::Future<
firebase::functions::
HttpsCallableResult>
&future) {
OnCallCompleted(future,
completion_callback);
});
}
cout << "tid=" << pthread_self() << " -Cloud::Call(`" << name << "`, ...)" << endl;
return future;
}

firebase::Variant OnCallCompleted(
const firebase::Future &future,
std::function
completion_callback)
{
cout << "tid=" << pthread_self() << " +OnCallCompleted(...)" << endl;
firebase::Variant data;
auto status = future.status();
cout << "tid=" << pthread_self() << " OnCallCompleted: status=" << status << endl;
if (status == firebase::kFutureStatusComplete) {
auto httpcallresult = future.result();
auto error = future.error();
auto error_message = future.error_message();
if (error == 0) {
data = httpcallresult->data();
}
if (completion_callback) {
completion_callback(data, error, error_message);
}
}
cout << "tid=" << pthread_self() << " -OnCallCompleted(...)" << endl;
return data;
}

firebase::Future FooAsync(std::function completion_callback)
{
cout << "tid=" << pthread_self() << " +FooAsync(...)" << endl;
auto future = CallAsync("foo", firebase::Variant(),
[completion_callback](const firebase::Variant &data,
int error, const std::string &) {
int value = (error == 0) ? data.int64_value() : -1;
if (completion_callback) {
completion_callback(value);
}
});
cout << "tid=" << pthread_self() << " -FooAsync(...)" << endl;
return future;
}

int main() {
...
future = FooAsync([](int value) {
cout << "tid=" << pthread_self() << " FooAsync #3: value=" << value << endl;
FooAsync([](int value) {
cout << "tid=" << pthread_self() << " FooAsync #4: value=" << value << endl;
});
});
WaitForCompletion(future);
return 0;
}
```

When I run the code I get the following:
```bash
% ./FirebaseExample
WARNING: Database URL not set in the Firebase config.
DEBUG: Creating Firebase App __FIRAPP_DEFAULT for Firebase C++ 12.1.0
DEBUG: Validating semaphore creation.
DEBUG: Added app name=__FIRAPP_DEFAULT: options, api_key=..., app_id=..., database_url=, messaging_sender_id=..., storage_bucket=..., project_id=... (0x...)
Using Functions Emulator at 127.0.0.1:5001
...
tid=0x1fefccc00 +FooAsync(...)
tid=0x1fefccc00 +Cloud::Call(`foo`, ...)
tid=0x1fefccc00 Call: `foo` +Call(...)
DEBUG: Calling Cloud Function with url: 127.0.0.1:5001/functions-futures-async-nested/us-central1/foo
data: {"data":null}
tid=0x1fefccc00 Call: `foo` -Call(...)
DEBUG: Cloud Function response body = {"result":200}
tid=0x16b7fb000 +OnCallCompleted(...)
tid=0x16b7fb000 FooAsync #3: value=200
tid=0x16b7fb000 +FooAsync(...)
tid=0x16b7fb000 +Cloud::Call(`foo`, ...)
^C
```
The app appears to deadlock and never complete the future and print the expected `FooAsync #4: value=200`.
I have to Ctrl-C to end the app.

The above is simplified code for this Issue.
In my actual code I also show initializing the project app and successful multiple non-async and non-nested async calls.

#### Steps to reproduce:

100% repro with the code/steps listed above

#### Relevant Code:

https://github.com/DistroAV/firebase-functions-future-async-nested