stats: Add copy constructor to Distribution.
stats: Add copy constructor to Distribution.

The final stats copy was wrong when flow_histo overflowed.

Change-Id: I1fdd33eb7582a02e8f1726f4c9577531affc99b9

s-kanev/XIOSim,s-kanev/XIOSim,s-kanev/XIOSim,s-kanev/XIOSim