ovsdb: separate json cache for different monitor versions

ovsdb: separate json cache for different monitor versions

Cached json objects were reused when sending notifications to
clients. This created a problem when there were different versions
of monitors coexisting. E.g. clients expecting version2 notification
would receive messages with method == "update2" but payload in
version1 format, which end up failure of processing the updates.

This patch fixes the issue by including version in cache node.

Signed-off-by: Han Zhou <zhouhan@gmail.com>
Acked-by: Andy Zhou <azhou@ovn.org>
Signed-off-by: Andy Zhou <azhou@ovn.org>