Close JSON root context when the writer is closed

Close JSON root context when the writer is closed

We should be closing the root context when we are closed, not when
a its child is closed -- otherwise it would be impossible to write
multiple children of the root context.

Since we are in the area, also centralize some QName/ContainerNode
creation.

JIRA: YANGTOOLS-1029
Change-Id: I599fb5ad1ad8e36a441dff761984d5406e401d55
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>
(cherry picked from commit f8e3d9af24d8d5d1eeca6a714d29f9822537a773)