The topology coordinator executes several topology cmd rpc against some nodes
during a topology change. A topology operation will not proceed unless
rpc completes (successfully or not), but sometimes it appears that it
hangs and it is hard to tell on which nodes it did not complete yet.
Introduce new REST endpoint that can help with debugging such cases.
If executed on the topology coordinator it returns currently running
topology rpc (if any) and a list of nodes that did not reply yet.
(cherry picked from commit c8ce9d1c60)
ad91198417