Description: We recently added a connection pool map class which maps keys to connection pools. A new key means a new connection pool. To prevent unbounded growth of connection pools, we placed a limit on the pool. Allocating beyond the limit causes the map to attempt to reclaim unused pools prior to allocating a new one. If none can be reclaimed, the map returns a failure.

We did not add the ability to configure the limit; its default was unlimited. This commit adds the ability to configure a limit by extending the cluster circuit breaker thresholds with a new optional configuration field: max_connection_pools.

Worth noting:

1. The default if no configuration is set is still unlimited.
2. The configuration is per priority, like the other thresholds.
3. The per priority configuration required some small refactoring: I added a class which creates one connection pool map per priority, so that each connection pool map can track its resources independently.
4. I added an extremely simple integration test which shows the behaviour when the limit is reached.

Risk Level: Medium
We're in the main code path for http. But, the default behaviour should not have changed, so there shouldn't be any major impact.

Testing:
Unit testing + integration testing
I spun up a local envoy instance, showing that:

1. When the limit was reached, I got a 503 back
2. If a pool was idle, even though I was at the limit, an old pool would be recycled and the request would go through.

Docs Changes: Protobuf docs + additions to the circuit breaking section in the overview.

Release Notes: Added a release not for the new circuit breaker.