messages: remove `using namespace std` in headers

Avoid polluting global namespace: .cc files should
do the `using` if they want it.

Signed-off-by: John Spray <john.spray@redhat.com>
mds: remove `using namespace std` in headers

Avoid polluting global namespace: .cc files should
do the `using` if they want it.

Signed-off-by: John Spray <john.spray@redhat.com>