ProxyStepHandler does not include final result of remote operation in its response

ProxyStep handler includes the ProxyOperationControl.operationPrepared value in its response and then commits/rolls back the remote transaction, but then instead of using the operationCompleted value it just has

// TODO what if the response on the proxy changed following the operationPrepared callback?
// e.g. the rolled back flag? This may all be ok, just needs to be thought about, tested.
// If it proves unnecessary, get rid of ProxyOperationControl

This breaks calls through from the master HC to a slave that the slave then handles as a multi-server op (e.g. /host=slave/system-property=foo:add(value=bar)) because the multi-server response is not included in operationPrepared, only in operationCompleted.