Removed unused variable and use std::atomic when communicating with unix signal handler

The variable my_sig_num_ was the only place where getSigNum was ever called and the value of the variable was never used after that. Both the variable and the function have been removed.
The unix signal handler code no longer uses a mutex and instead uses a std::atomic to denote if an external user has requested that the framework stop processing events.
Switch from using a mutex to using a std::atomic when signaling shutdown request

To be properly thread safe under C++11 we switched from using a volatile bool to a volatile std::atomic<bool> as the way to communicate an external shutdown request. The mutex usr2_lock was also removed since it actually was never used correctly anyway and the std::atomic does the work the mutex should have been used for.
The function getSigNum() and its related global variables were removed since the function is no longer used anywhere.
Use a faster form of synchronization when checking for shutdown request

Since the variable shutdown_flag is not being used to synchronize other variables we can use the memory_order_relaxed which requires less synchronization between CPUs.
Merge pull request #715 from Dr15Jones/signalHandlersUseAtomics

Signal handlers use atomics