[FLINK-5442] Ensure order of enum elements in StateDescriptor.Type

[StefanRRichter]

This PR adds a test to ensure the order of elements in enum StateDescriptor.Type which is important for the serialization format.

[StefanRRichter]

cc @StephanEwen

[StephanEwen]

Looks good, but is that not breaking it now, by changing the order?
Or is it only breaking between 1.2-RC0 and 1.2RC1?

I would almost suggest to not reorder now - it appears users have jumped onto the RC0 already and this is not really a crucial fix (more a code beautification)

[StefanRRichter]

It only breaks w.r.t. to the previous dev master branch, right? I just preferred to have UNKNOWN at position 0 because the future way of adding enum entries is basically "append only" and having this special state in the middle then would look weird.

[StephanEwen]

When was that enum introduced? Can we get a feeling how many people are already using this?
There are quite some users on the dev branch.

[StefanRRichter]

It was introduced by middle/end of december 16. The benefit is also just for esthetic reasons. Would you like me to restore the old order?

[StephanEwen]

If it is in so shortly, lets make it clean and keep the new order...

[StephanEwen]

Merging this...