All uses of beast::Thread were previously removed from the code
base, so beast::Thread is removed. One piece of beast::Thread
needed to be preserved: the ability to set the current thread's
name. So there's now a beast::CurrentThreadName that allows the
current thread's name to be set and returned.
Thread naming is also cleaned up a bit. ThreadName.h and .cpp
are removed since beast::CurrentThreadName does a better job.
ThreadEntry is also removed, but its terminateHandler() is
preserved in TerminateHandler.cpp. The revised terminateHandler()
uses beast::CurrentThreadName to recover the name of the running
thread.
Finally, the NO_LOG_UNHANDLED_EXCEPTIONS #define is removed since
it was discovered that the MacOS debugger preserves the stack
of the original throw even if the terminateHandler() rethrows.
