Firestore emulator freezes/fails when doing document join using block()

### Environment info
> firebase --version
10.2.1
Windows 10

### Test case

https://github.com/VitorBittencourt/transaction-error.git

### Steps to reproduce

Create at least 1 document by POSTing to each of the following:
localhost:8080/foo/save/123
localhost:8080/bar/save/123

Then for a working non-blocking version, GET the join of the documents at:
localhost:8080/bar/join/123

Then GET
localhost:8080/bar/blockjoin/123

### Expected behavior

Both blocking and non-blocking versions should return a collection like:
```
[
{
"localIdentifier": "123",
"fooCountList": [
2,
4,
3,
1,
0
],
"barCount": 1
},
{
"localIdentifier": "123",
"fooCountList": [
2,
4,
3,
1,
0
],
"barCount": 2
}
]
```

### Actual behavior

The non-blocking version, that doesn't rely on Flux.block(), works as expected.

The blocking version (I know it's not good code, but it is what I had at the time) **works fine against the real Firestore**, but when used on the emulator throws exceptions like

> 2022-Jun-09 15:50:34.467 [grpc-nio-worker-ELG-1-2] ERROR o.s.b.a.w.r.e.AbstractErrorWebExceptionHandler - [ee0dcac8-2] 500 Server Error for HTTP GET "/bar/blockjoin/123"
> java.lang.IllegalStateException: Timeout on blocking read for 5000000000 NANOSECONDS
> at reactor.core.publisher.BlockingSingleSubscriber.blockingGet(BlockingSingleSubscriber.java:123)
> Suppressed: reactor.core.publisher.FluxOnAssembly$OnAssemblyException:
> Error has been observed at the following site(s):
> |_ checkpoint ⇢ Handler com.example.transactionerror.BarController#blockingJoin(String) [DispatcherHandler]
> |_ checkpoint ⇢ HTTP GET "/bar/blockjoin/123" [ExceptionHandlingWebHandler]
> Stack trace:
> at reactor.core.publisher.BlockingSingleSubscriber.blockingGet(BlockingSingleSubscriber.java:123)
> at reactor.core.publisher.Mono.block(Mono.java:1730)
> at com.example.transactionerror.BarController.lambda$blockingJoin$4(BarController.java:69)
> ...

Clone the linked transaction-error reproduction and run it with Firebase CLI 10.2.1 on Windows 10. POST to /foo/save/123 and /bar/save/123, then compare GET /bar/join/123 with /bar/blockjoin/123; done means the blocking endpoint returns the expected collection without freezing or timing out.

Written by the indexing model from the issue text.