Utility sstablesplit should prevent users from running when C* is running

The sstablesplit utility is designed for use when C* is offline, but there is nothing stopping the user from running it on a live system. There are also no warning messages alerting the user to this effect.

The help information should also be updated to explicitly state that the utility should only be used when C* is offline.

Finally, this utility is not included in any of the documentation. Please update accordingly. Thanks.