Enable static & dynamic configuration of Redis connection pool size

What does this MR do?

We currently use hard coded values for configuring redis connection pools at runtime.

Rather than relying on hard-coded constants, we now first try to read a user specified value for the connection pool size from resque.yml. If that does not exist, we fall back to set a sensible default based on the current environment's concurrency settings.

To that end, two changes were made:

1. In resque.yml, where the remaining redis config resides, a new option can be specified (connection_pool_size), for puma, unicorn and sidekiq. Whenever this config is present, it takes effect as-is.
2. If no user specified values are supplied, we default to a pool size of concurrency + 50% (concurrency meaning the number of worker threads per process). The added 50% are to account for other non-worker threads trying to connect to Redis (this is just a theoretical maximum; connections in the pool are created lazily so this cap comes essentially free). I chose a factor instead of adding a constant so that the pool size scales with the concurrency level of the runtime. It's just a heuristic though; as long as we don't see clients timing out while waiting for connections to become available, we're good.

TODO:

• Split up pool size config into web specific and job specific work loads
• Apply a factor for extra head room
• do not pass along non-Redis client cfg from what we parse from resque.yml

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Style guides

Availability and Testing

Upon deploying this, the behavior compared to now would change as follows:

• Unicorn worker Redis pool size will shrink from 5 to round(1.5 * 1) (= 2)
• Sidekiq worker Redis pool size will change from 5 + num_threads to round(1.5 * num_threads)
• Puma worker Redis pool size will change from 5 to 1.5 * num_threads

This is assuming that we do not provide a default for connection_pool_size in resque.yml. Otherwise all cases collapse to using the specified connection_pool_size for the given runtime.

Closes #35559 (closed)