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.
Log thread name and exception type on unhandled exceptions and use a
terminate handler to get a stack trace that includes the function that
thows the exception.
