FIX: Always clear caches after committing the current transaction (#22491)

Instead of having to remember every time, just always wait until the
current transaction (if it exists) has committed before clearing any
DistributedCache.

The only exception to this is caches that aren't caching things from
postgres.

This means we have to do the test setup after setting the test
transaction, because doing the test setup involves clearing caches.

lib/distributed_cache.rb

@@ -19,4 +19,12 @@ class DistributedCache < MessageBus::DistributedCache
     self.defer_set(k, value)
     value
   end
+
+  def clear(after_commit: true)
+    if after_commit
+      DB.after_commit { super() }
+    else
+      super()
+    end
+  end
 end