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/basics/impl/Sustain.cpp

@@ -19,7 +19,7 @@
 
 #include <BeastConfig.h>
 #include <ripple/basics/Sustain.h>
-#include <ripple/basics/ThreadName.h>
+#include <ripple/beast/core/CurrentThreadName.h>
 #include <boost/format.hpp>
 
 // For Sustain Linux variants
@@ -103,7 +103,7 @@ std::string DoSustain ()
         auto cc = std::to_string (childCount);
         if (pChild == 0)
         {
-            setCallingThreadName ("main");
+            beast::setCurrentThreadName ("rippled: main");
             signal (SIGINT, SIG_DFL);
             signal (SIGHUP, SIG_DFL);
             signal (SIGUSR1, SIG_DFL);
@@ -111,7 +111,7 @@ std::string DoSustain ()
             return "Launching child " + cc;
         }
 
-        setCallingThreadName (("#" + cc).c_str());
+        beast::setCurrentThreadName (("rippled: #" + cc).c_str());
 
         sleep (sleepBeforeWaiting);