basic scope implementation #2629

* scope-impl

* refine GetOrCreateVariable

* refine code of scope with style check

* refine code of operator

* clang format

* move code to scope.h, remove scope.cc

* add Create for scope

* merge CreateVar and GetOrCreateVar

* do not use default argument in Scope

* update design doc

 Init partial network parameters from another saved model.

* add trainer library

* modifty file name

* move trainer to master client

* update

* update

* modify monitor master to receive a chan

* update

* use etcd client from etcd_client.go

* update

* update

* remove etcd client without lock

* update

* update the comment

* update commonts

fix xmap_readers and refine flowers dataset

Adding Enforce to platform

Add paddle::string::{F,S,}Printf

"fix client send empty gradients bug"

Rename paddle::strings::StringPiece into paddle::string::Piece

Memory CPU allocator

Make pserver able to get server index without etcd (decouple pserver with etcd)

Remove must_check in paddle::platform

1. rename 'useXmap' to 'use_xmap'
2. add comments about exchanging train data and test data

Fix bug in MultiGradientMachine.

Basically from caffe2::logging.h, but only expose `PADDLE_ENFORCE`
interface.