Moved common structures out of cta:: into cta::common::sub-namespaces.

common/admin/AdminHost.cpp

common/admin/AdminHost.hpp

common/admin/AdminUser.cpp

common/admin/AdminUser.hpp

common/archiveNS/ArchiveDirEntry.cpp

common/archiveNS/ArchiveDirEntry.hpp

common/archiveNS/ArchiveDirIterator.cpp

common/archiveNS/ArchiveDirIterator.hpp

common/threading/Threading.hpp

nameserver/CastorNameServer.cpp

nameserver/CastorNameServer.hpp

nameserver/CastorNameServerTest.cpp

nameserver/NameServer.hpp

nameserver/mockNS/MockNameServer.cpp

nameserver/mockNS/MockNameServer.hpp

nameserver/mockNS/MockNameServerTest.cpp

nameserver/mockNS/makeMockNameServerBasePath.cpp

scheduler/DummyScheduler.cpp

scheduler/DummyScheduler.hpp

scheduler/OStoreDB/OStoreDB.cpp