Forced retry


In addition to automatic retry upon validation failure, code should be able to force it, e.g. by calling NstmMemory.Retry() oder throwing a special exception, e.g. NstmRetryException.
However, the retry should then not be immediate! It should happen not before some of the data accessed by the tx has changed. Otherwise its outcome would be no different. Facilities need to be introduced to have transactions waiting for a retry notified of such changes. A tx waiting for a retry needs to register itself with all accessed tx values.