Fix #953 make _end_log and _begin_log non static data members in log4tango

Replace the _end_log and _begin_log construct with static data members with more of a tag construct. It integrates better with windows where static data member must be exported with __declspec on top of using the EXPORT_ALL_SYMBOLS cmake variable on compilation. the new _end_log and _begin_log could be inlined but c++17 support is required, it shouldn't cause much trouble if they're not though.