Apply staged CVM changes

Applies every staged change as one PATCH /cvms/{cvm_id}: one VMM update and at most one restart; a stopped CVM stays stopped. Rejected with ERR-03-053 when the CVM's compose changed since the first change was staged. Accepts the same X-Compose-Hash, X-Transaction-Hash and X-Prepare-Only headers and returns the same responses as PATCH /cvms/{cvm_id}; the staged changes are kept until an apply dispatches them. A phase 2 apply is rejected with ERR-03-054 when changes were staged after its phase 1; apply again without the headers to prepare them. An update committed through a commit token (POST /cvms/{cvm_id}/commit-update) does not clear them: the next apply is then rejected with ERR-03-053, and the staged changes must be discarded.