Make DriveHandler testable and create Unit Tests for this class

ReleaseNotes.md

common/log/StringLogger.hpp

scheduler/IScheduler.hpp

scheduler/Scheduler.hpp

tapeserver/castor/messages/CMakeLists.txt

...ver/castor/tape/tapeserver/daemon/DataTransferSession.hpp

tapeserver/daemon/CMakeLists.txt

tapeserver/daemon/DriveHandler.cpp

tapeserver/daemon/DriveHandler.hpp

tapeserver/daemon/DriveHandlerProxy.cpp

tapeserver/daemon/DriveHandlerProxy.hpp

tapeserver/daemon/DriveHandlerStateReporter.cpp

tapeserver/daemon/DriveHandlerStateReporter.hpp

tapeserver/daemon/DriveHandlerTests.cpp

tapeserver/daemon/ProcessManager.cpp

tapeserver/daemon/ProcessManager.hpp

tapeserver/daemon/SubprocessHandler.hpp

tapeserver/daemon/TapeDaemon.cpp

tapeserver/daemon/TapeDaemon.hpp

tapeserver/daemon/TapedProxy.cpp