Checkpoint buffer is flushed prematurely when another bundle is started during checkpointing

As part of a checkpoint, the current bundle is finalized. When the bundle is finalized, the watermark, currently held back, may also be progressed which can cause the start of another bundle. When a new bundle is started, any to-be-buffered items from the previous bundle for the pending checkpoint may be emitted. This should not happen.

This only effects portable pipelines where we have to hold back the watermark due to the asynchronous processing of elements.