1. Added rollbackTransaction to AbstractKeyValyeStoreOutputOperator that extending operators will implement to process rollback.
2. Added a continueOnError property to AbstractKeyValueStoreOutputOperator which when set will cause the operator to continue processing future windows even if there is an error in the transaction for the current window.
3. Added timeout property to RedisOutputOperator for specifying redis connection timeout which an application can set.
4. Implemented rollbackTransaction to discard an existing transaction as it is called when there is an error.
Fixes #5 #6 #7