* `get_upgrade` => `GET _upgrade` -- Return the status
* `upgrade` => `POST _upgrade` -- Perform the operation
Original specification part of c021f22523.
Related: #7884, #7922
This commit does the following:
* Add the new API at the rest layer, being backed by the optimize API
with upgrade flag, and segments api to find upgrade status.
* Add `upgrade` flag to optimize API, and deprecate `force` flag (will
remove in master)
* Add test for both synchronous and async upgrade
closes#7884closes#7922