IDL index should be able to be used without having to be in a
transaction. However, current implementation leads to crash if
a reference type column is being set in an index row for querying
purpose when it is not in a transaction. It is because of the
uninitialized arcs and unnecessary updates of the arcs. This patch
fixes it by identifying index rows by a magic uuid, so that when
parsing index row, the arcs are not updated. A new test case is
added to cover this scenario.

Signed-off-by: Han Zhou <zhouhan@gmail.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>