Convert the raylet client (the code in local_scheduler_client.cc) to proper C++. (#3511)

* refactoring

* fix bugs

* create client class

* create client class for java; bug fix

* remove legacy code

* improve code by using std::string, std::unique_ptr rename private fields and removing legacy code

* rename class

* improve naming

* fix

* rename files

* fix names

* change name

* change return types

* make a mutex private field

* fix comments

* fix bugs

* lint

* bug fix

* bug fix

* move too short functions into the header file

* Loose crash conditions for some APIs.

* Apply suggestions from code review

Co-Authored-By: suquark <suquark@gmail.com>

* format

* update

* rename python APIs

* fix java

* more fixes

* change types of cpython interface

* more fixes

* improve error processing

* improve error processing for java wrapper

* lint

* fix java

* make fields const

* use pointers for [out] parameters

* fix java & error msg

* fix resource leak, etc.

java/runtime/src/main/java/org/ray/runtime/RayNativeRuntime.java

@@ -61,7 +61,7 @@ public final class RayNativeRuntime extends AbstractRayRuntime {
     // Load native libraries.
     try {
       resetLibaryPath();
-      System.loadLibrary("local_scheduler_library_java");
+      System.loadLibrary("raylet_library_java");
       System.loadLibrary("plasma_java");
     } catch (Exception e) {
       LOGGER.error("Failed to load native libraries.", e);