map_difference: Allow on unordered_map
map_difference: Allow on unordered_map

This patch changes the map_difference interface so difference()
can be called on on unordered_maps.

Signed-off-by: Duarte Nunes <a60e8b7496f7715d71354fe4c06dd24017025bf1@scylladb.com>

scylladb/scylla,raphaelsc/scylla,scylladb/scylla,scylladb/scylla,raphaelsc/scylla,avikivity/scylla,kjniemi/scylla,raphaelsc/scylla,duarten/scylla,avikivity/scylla,duarten/scylla,scylladb/scylla,kjniemi/scylla,avikivity/scylla,duarten/scylla,kjniemi/scylla