The concurrency limit, because it uses a semaphore, had unfortunate
interactions with the use of `call_all` in the polling monitor.
`CallAll` can internally have requests which are holding the semaphore,
so it must be polled regularly to check on those requests.
If the task holding the `CallAll` hangs on some other future, that
can lead to a deadlock.

In our case, it can hang in the `response_sender.send((id, response)).await`,
if the channel is full and the subgraph runner takes a long time to check it.
This could be fixed, but the footgun would remain so it seemed best to
rely only on the rate limit which can't deadlock.