feat(cbrs): load based routing strategy

[xurui-c]

Basically does what OutcomesBasedRoutingStrategy does except when cluster load is low enough, we let the query through even if allocation policies say no

Rollout plan:

1. only deploy this strategy for org 1 (Sentry) via Snuba Admin by setting storage_routing_config_override = {'{"1": {"version": 1, "config": {"LoadBasedRoutingStrategy": 1.0}}}',
2. deploy to 50% of our customers via setting default_storage_routing_config = '{"version": 1, "config": {"LoadBasedRoutingStrategy": 0.5, "OutcomesBasedRoutingStrategy": 0.5}}'