Implement load balancer feature controls

What and Why

Since we need to deliver improvements to the database load balancer as part of Load Balancer Improvements (&10703), and since feature flags involve the database, making them unsuitable for controlling load balancer features, we need to implement a mechanism to roll out changes to the load balancer itself.

To do this, we will bypass the load balancer during feature flag checks.

The rollout of which will be controlled with an environment variable.

Deliverables

• MR to bypass the load balancer during feature flag checks - !137319 (merged)
• Enable the FEATURE_FLAGS_BYPASS_LOAD_BALANCER environment variable in staging, and verify that all load balancer checks go to the primary.
• Enable the FEATURE_FLAGS_BYPASS_LOAD_BALANCER environment variable in production, and verify that all checks go to the primary.
• Remove the FEATURE_FLAGS_BYPASS_LOAD_BALANCER environment variable, defaulting to the new, all-primary, code path.

/cc @alexives @rogerwoo