Created by: typhoonzero

Changes:

• variable response do not hold scope, it use a callback to get variable, or hold the var ref.

• handlers have full control of what the request should be processed on server.

• move handlers to one folder.

• sync handlers and async handlers are different handlers

• grpc_request.h have only one class

• out_var do not use a pointer of pointer.

• A barrier class

• rpc_server only hold send_barrier and recv_barrier, do not hold one barrier for each rpc type

• rpc_server hold one barrier for each marked variable to mark variable ready state for get and barrier all workers.

• README for sync server scheme

• TODO: will update brpc folder in another PR