ofproto-dpif: Improve RSPAN translation performance from O(n**2) to O(n).

ofproto-dpif: Improve RSPAN translation performance from O(n**2) to O(n).

This code previously checked whether each individual mirror output was
already in the set of destinations.  This is O(n**2) in the number of
ports in a bridge.

The new code uses a smarter algorithm to eliminate duplicates, one that is
O(n) in the number of ports in a bridge.