ovn: Use callback function instead of simap for logical port number map.

ovn: Use callback function instead of simap for logical port number map.

An simap is convenient but it isn't very flexible.  If the client wants to
keep extra data with each node then it has to build a second parallel data
structure.  A callback function is kind of a pain for the clients from the
point of view of having to write it and deal with auxiliary data, etc., but
it allows the storage to be more flexible.

An upcoming commit will make further use of this capability.

Signed-off-by: Ben Pfaff <blp@ovn.org>
Acked-by: Justin Pettit <jpettit@ovn.org>