ofproto: Recycle least recently used ofport.

ofproto: Recycle least recently used ofport.

If there is a lot of churn in creation and deletion of
interfaces, we may end up recycling the ofport value of a
recently deleted interface for a newly created interface.
This may result in an old stale openflow rule applying
on the newly created interface.

With this commit, when a new port is added, try and provide
it an ofport value that has not been used during the current
run. If such value does not exist, provide the least
recently used ofport value.

Signed-off-by: Gurucharan Shetty <gshetty@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>