[FLINK-4445] Add option to ignore unmapped checkpoint state

[uce]

When restoring from a checkpoint/savepoint, state for each operator has to be restored. For savepoints, this means that the user cannot remove an operator from her topology and still use the savepoint.

With this change, we will allow to ignore state that cannot be mapped back to the job being restored. The default behaviour does not change.

Changes

• I've removed the allOrNothingState flag as it was only effecting non-partitioned operator state and never set to true anyways (except tests). The flag controlled whether each non-partitioned operator state was restored.
• Moved the savepoint path from the JobSnapshottingSettings to the JobGraph
• Added the --ignoreUnmappedState (short -i) flag to the run command: bin/flink run -s <savepointPath> -i ...

I've tested this manually by triggering a savepoint for a job, adjusting the job (removing an operator), and then trying to resume from the savepoint. By default, restoring fails, but with the flag everything works.

[StefanRRichter]

+1, the proposed changes look good to me.

[uce]

Thanks for the review. Going to merge this.

[StephanEwen]

Looks good to me, except the name ignoreUnmappedState ;-)
As usual, the three most difficult things in computer science are (1) finding good names, and (2) off-by-one errors.

What do you think about calling something like allowUnresumedState or allowNonRestoredState? The "allow" to me implies that this is a valid scenario.

[uce]

Very much agree Stephan! I don't know what sounds better to native speakers and more intuitive to users though... unresumed state or non restored state? ;-) @greghogan @jgrier do you have any input on this?

The internal behaviour is the following: The checkpoint/savepoint stores state for each operator of the original job graph (from which the checkpoint/savepoint was triggered) keyed by the operator ID. When a user resumes from this checkpoint/savepoint, the checkpoint coordinator tries to map each state (keyed by operator ID) to the operators of the new job. This PR allows ;-) that some of this state is not restored. Any ideas on how to call this?