Don't replicate SETSLOT to nodes that may crash

[zuiderkwast]

#245 replicates CLUSTER SETSLOT. Old replicas would generate an error reply for this. They would even panic if 'propagation-error-behaviour' config is set to 'panic'.

To prevent that, use a flag in clusterMsg as a capability flag to indicate support for this feature. If the replica doesn't have this flag set, we don't replicate SETSLOT to it.

It'd be similar to the flag CLUSTER_NODE_EXTENSIONS_SUPPORTED.

Originally discussed here: #245 (comment)_