Remove beast::Thread (RIPD-1189):

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.

src/ripple/core/impl/JobQueue.cpp

@@ -461,7 +461,6 @@ JobQueue::processTask ()
             }
             type = job.getType();
             JobTypeData& data(getJobTypeData(type));
-            beast::Thread::setCurrentThreadName (data.name ());
             JLOG(m_journal.trace()) << "Doing " << data.name () << " job";
             on_dequeue (job.getType (), start_time - job.queue_time ());
             job.doJob ();