Update namespace replication task cleanup logic with connected clusters

[yux0]

Expected Behavior

calculate namespace replication ack level by active cluster connections

NS replication metadata: Cluster A (disconnected): 100 Cluster B (connected): 1000 Cluster C (connected): 500

minimum ack level: 500, delete all tasks before 500.

Actual Behavior

NS replication metadata: Cluster A (disconnected): 100 Cluster B (connected): 1000 Cluster C (connected): 500

minimum ack level: 100

Specifications

In 1.16, we allow remove clusters from namespace cluster list. We should update this logic accordingly.