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/app/ledger/impl/LedgerCleaner.cpp

@@ -22,9 +22,8 @@
 #include <ripple/app/ledger/InboundLedgers.h>
 #include <ripple/app/ledger/LedgerMaster.h>
 #include <ripple/app/misc/LoadFeeTrack.h>
-#include <ripple/core/ThreadEntry.h>
 #include <ripple/protocol/JsonFields.h>
-#include <ripple/beast/core/Thread.h>
+#include <ripple/beast/core/CurrentThreadName.h>
 
 namespace ripple {
 namespace detail {
@@ -243,13 +242,7 @@ private:
 
     void run ()
     {
-        threadEntry (
-            this, &LedgerCleanerImp::runImpl, "LedgerCleanerImp::run()");
-    }
-
-    void runImpl ()
-    {
-        beast::Thread::setCurrentThreadName ("LedgerCleaner");
+        beast::setCurrentThreadName ("LedgerCleaner");
         JLOG (j_.debug()) << "Started";
 
         init();