Need ways to check whether segment reload/refresh is done on server side

Currently segment reload/refresh is done asynchronously. There is no way for Pinot admin to know whether the segment reload/refresh request is fully done on server side. It could be possible that multiple reload/refresh are made, which can cause race conditions.

We need some way to figure out the state of the reload/refresh request on server side.

Some thoughts:
* Add server level APIs to fetch the state.
* Add callback when the request is done.

Start by tracing the server-side segment reload/refresh request path and reviewing existing server-level admin APIs. Compare an API for request state with a completion callback, then define how an administrator can tell that the request is fully complete and how repeated requests avoid race conditions.

Written by the indexing model from the issue text.