Use a DDL subscriber to handle updates to the stats meta

[Rustin170506]

Enhancement

We introduced a DDL notifier when developing the new priority queue to deliver DDL events in a highly durable manner.

However, the current stats meta handling still relies on an unreliable in-memory channel. Therefore, it would be better to use the DDL notifier to handle stats meta updates as well.

I tried this before, as seen in this PR. However, many of our test cases rely on the behavior where the unit tests do not handle DDL events by default, which caused a lot of test failures.

This time, I will keep the in-memory channel for testing purposes only. This will help reduce the effort required to fix all the broken tests.