Fix Match intersection logic

    - Change in Match.java: Empty match represents all packets (universal set) while empty set is the null match.
                        So, in case of no intersection, return a null Match.
                            intersect() and getIntersection() optimized in case of universal and empty sets
    - Changes in MatchTest.java to junit the above changes
    - Change in FlowEntry.java: Match.mergewithFilter() is now returning a new Match instance

Change-Id: Idd3ff80d2fbcc11c3f83757268806596b7b2fc50
Signed-off-by: Alessandro Boch <aboch@cisco.com>