Moved DirIterator from the scheduler directory to common

common/CMakeLists.txt

common/DirEntry.cpp

common/DirEntry.hpp

common/DirIterator.cpp

common/DirIterator.hpp

nameserver/MockNameServer.hpp

nameserver/NameServer.hpp

scheduler/CMakeLists.txt

scheduler/FileSystemDirEntry.hpp

scheduler/MockSchedulerDatabase.cpp

xroot_plugins/XrdProFile.cpp