map_difference: accept std::unordered_map
map_difference: accept std::unordered_map

map_difference implementation doesn't need elements in the container to
be sorted.

Signed-off-by: Paweł Dziepak <e29bb3265626c60caf76d587cabdfcd8b379ef80@scylladb.com>

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