Use different cache namespaces for different tenants

Since records in different tenants can have the same ID, they can share
the same `cache_key`, and so we need a namespace to differentiate them.
Without them, records from one tenant could expire the cache of a record
from another tenant.

config/environments/staging.rb

@@ -58,7 +58,7 @@ Rails.application.configure do
   config.log_tags = [:request_id]
 
   # Use a different cache store in production.
-  config.cache_store = :mem_cache_store
+  config.cache_store = :mem_cache_store, { namespace: proc { Tenant.current_schema }}
 
   # Use a real queuing backend for Active Job (and separate queues per environment).
   # config.active_job.queue_adapter     = :resque