The two internal actors are replaced by two internal states.

Their execution is not concurrent anymore,
but we the code and the flow is simplified and we avoid some race conditions.

The control plane is not getting the update notify signal through the
broker anymore.

The Notify RPC is entirely removed.

Most of the test in the indexer scheduler become Control plane tests.

The RefreshPlan loop is not necessary since after we moved to
the control plane as a metastore proxy world, the control
plan cannot miss any Index/Source event.