CFR: `ctk cfr sys-export` stalls after 24% ...

# What happened?
Running `sys-export` on a larger cluster seems to stall and make the node co-ordinating the queries _un-responsive_. Looking at the created report files it is not clear where it exactly got stuck. CTRL+C did not terminate the command, had to fire a `kill` in the shell to stop the python process.

# To discuss/Implement

- [ ] THROTTELING: Think about a possible throttling, especially for clusters with a large amount of jobs, shards, ...
- [ ] TIMEOUT: set an explicit _statement timeout_ to avoid long running, blocking queries (althought not 100% whether that also works for queries on _sys tables_
- [ ] SHARDS: for troubleshooting purpose it might suffice to report the _not started_ shards, same for the sys.allocations query. In case we fancy to have to complete list of shards, this could be moved to a cli arg, where the default is to report only _not started_ shards.