feature request: add cluster intialization status to admin `/init_dump`

We can now query the active cluster and warm cluster to know what clusters are able to use.
It's adequate to explain why a request is "NR 503", but it's still hard to troubleshooting why a cluster stays warming up.

We can expose the per cluster init::manager to cluster manager and query init manager's unready target.

The followings are need to be resolved.
1. The cluster ready details query should be exposed to admin interface `/init_dump`
2. Not all the blocking items are under the frame of init::manager. If each of the blocking item attaches to a init::target with a meaningful name, the init status can be as simple as listing all the unready target names. ~The current cluster initialization does not end with init::manager announce "ready".~

Thoughts?