ovn-controller: Add data structure for indexing lports, multicast groups.

ovn-controller: Add data structure for indexing lports, multicast groups.

This was more or less implemented inside lflow.c until now, but some
upcoming code that shouldn't be in that file needs to use it too.

This also adds a second index on lports, so that lports can be looked up
based on the logical datapath tunnel key and the logical port tunnel key.
An upcoming commit will add a user for this new index.

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