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.

python/ray/utils.py

@@ -69,7 +69,7 @@ def push_error_to_driver(worker,
     if driver_id is None:
         driver_id = ray_constants.NIL_JOB_ID.id()
     data = {} if data is None else data
-    worker.local_scheduler_client.push_error(
+    worker.raylet_client.push_error(
         ray.ObjectID(driver_id), error_type, message, time.time())