ovsdb-idl: Use modern form of <monitor-requests>.

ovsdb-idl: Use modern form of <monitor-requests>.

Long ago, a <monitor-requests> object in the OVSDB protocol mapped a table
name to a single <monitor-request>.  Since then, it has mapped a table name
to an *array of* <monitor-request> objects, but the OVSDB IDL has never
been updated to use the modern form.  This commit makes that change.

Reported-by: Anil Jangam <anilj.mailing@gmail.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>
Acked-by: Justin Pettit <jpettit@ovn.org>