Skip savepoints for recovery

Intermediate savepoints should not be used for recovery. In order to achieve that we should:

• do not send notifyCheckpointComplete for intermediate savepoints
• do not add them to CompletedCheckpointStore

Important! Synchronous savepoints (stop-with-savepoint) should still commit side-effects. We need to distinguish them from the intermediate savepoints.

https://cwiki.apache.org/confluence/x/bIyqCw#FLIP193:Snapshotsownership-SkippingSavepointsforRecovery

Document recommendation to drop (change UID) the external state (like transactions from two phase commit sinks) if starting multiple jobs from the same intermediate savepoint.