mod_cluster proxy operations' rollback handlers make potentially dangerous assumptions

The current implementation of certain mod_cluster proxy operations are registering potentially dangerous rollback handlers. They are making an assumption what was the state of the system prior to the operation.

For instance, if the proxy was already disabled, and a disable operation is called which intermittently fails, a callback handler will be called which will enable the proxy. This is dangerous as attempting to disable an already disabled proxy may potentially enable it.

This behavior should be removed.