Guarded log functions with mutex to prevent threading errors (issue #569)

This PR fixes issue #569 (closed) by using a mutex in the begin_event, end_event functions. Since I could not find anywhere which C++ version is the minimum supported one, I wrote a custom guard class based on pthread instead of using the one provided with C++11. The fix was used in a large TTCN project (> 10.000 LOC TTCN) and resolved the issue.