Provide a configuration to configure total max event poolsize (cumulative count over horizontal scaling)

### Is your proposal related to a problem?

AFAIK the current the pool size configuration means `Max event processing threads` and this is per instance, and when Hasura runs horizontally this value could get multiplied.

#### What can go wrong?

* Suppose If I have a event stream that goes down for some time
* and when it comes back It could have significant load on the Hasura event queue
* I would expect Hasura to unload that without having more than 100 (or whatever the poolsize) request at the same time.
* Now if I have Hasura running horizontally (may be even with an auto scale setup) this could load my event handler in unexpectedly high rate.
* (If I have 3 hasura instances up, Hasura would have 300 simultaneous requests, which I may not expect my RHS system to handle)
* The event handler that I have in the RHS may not be designed to handle that much load, this could cause errors

### Describe the solution you'd like

There should be a way to configure cumulative total max http poolsize instead of the event pool size. (just like `total_max_connections`)

### Describe alternatives you've considered

If I have control over RHS, probably I can load balance RHS system or event increase the resources, but considering the expectation is to have Hasura manage the queue, this may not be a good solution.

No files, tests, or entry points are named. Start by locating the current event pool-size configuration and the existing total_max_connections handling, then trace how event processing behaves across horizontally scaled instances. Done means a cumulative maximum can be configured and prevents the event handler from receiving more concurrent requests than that limit.

Written by the indexing model from the issue text.