Add flush() to IFrameWriter This method is expected to be used with feeds to push frames all the way to storage when needed. As of now, it is needed in two cases: 1. No activities in ingestion node and need to push content so it can be stored. 2. When the ingestion node needs to move the checkpoint ahead if the at least once semantics are used.
Two feeds make use of this function. The filesystem feed and couchbase feed which was introduced as well in this change.