add verbose logging during index close
[SPEC] Separated the "Upgrade Index" API into two methods

* `get_upgrade` => `GET _upgrade`  -- Return the status
* `upgrade`     => `POST _upgrade` -- Perform the operation

Original specification part of c021f22523a69583b94c56921792b727ab4030fb.

Related: #7884, #7922
[CORE] Log if CircuitBreaker is tripping

Today we only throw an exception which might not be logged at all. This
commit adds debug logging if we are tripping a CB.

Closes #8050
Spec: Added human flag to indices.get_upgrade
Add link to new upgrade docs from indices page.

See #7922.
Add forgotten include for upgrade docs.

See #7922
Internal: fixed bw comp in ShardSearchTransportRequest

Original indices were introduced in 1.4.0.Beta1 as optional, preceded by a boolean flag that told whether they were empty or not. We have to keep serializing as optional only when reading and writing from/to 1.4.0.Beta1, although the original indices are not optional whenever we go and serialize the request they belong to (which is why the boolean got removed in 1.4.0).