Throw an error when repair on Time Window Compaction Strategy is called via jmx

[leonz]

Repair should never be used for TWCS tables. For the endpoints that we use via JMX, we should disable repair if the table happens to be TWCS.

Not sure if we should also disable via nodetool - it feels like we should keep that one for a "what if" (though I don't know what that "what if" is - like, we need repair to recover but are willing to take the storage hit).