Add SafeThread to fix thread JNI shutdown races.

During JVM shutdown, some JNI calls may not return, so it's not possible to
reliably perform a join() during static variable destruction (which occurs
as the JVM unloads the JNI module).

Also, due to static variable destruction, it's not safe to use any members
of a static class instance from a separate thread of execution.

SafeThread is a templated thread class and a related owner class that's
designed for safe operation and shutdown of threads in the presence of
callbacks that may not return.  It also passes ownership of variables from
the static instance to the thread, so the thread can safely operate until
it exits (the last operation of the thread being to destroy its instance).

Notifiers, RpcServer, and Logger now use SafeThread to ensure race-free
destruction in both C++ and Java.

All Java callback threads are now marked as Java daemon threads so they
don't keep the JVM running after main() terminates.

All Java callback threads are now named so their purpose is more easily
identified in a debugger.

Add SetRpcServerOnStart and SetRpcServerOnExit (similar to Listener).

include/ntcore_cpp.h

@@ -204,6 +204,9 @@ bool NotifierDestroyed();
  * Remote Procedure Call Functions
  */
 
+void SetRpcServerOnStart(std::function<void()> on_start);
+void SetRpcServerOnExit(std::function<void()> on_exit);
+
 typedef std::function<std::string(StringRef name, StringRef params)>
     RpcCallback;