mgr: use std namespace

mgr: use std namespace

This C++ code was relying on the Client.h header to bring in these names from
the std:: namespace. A subsequent commit plans to remove that header so add
namespace qualifier now.

Signed-off-by: Patrick Donnelly <pdonnell@ibm.com>
(cherry picked from commit 706b2be416246dd5923478d8a7ee015246f03e95)

Conflicts:
	src/mgr/DaemonServer.h: no header cleanup