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.

fix dim problem

The pserver need to have server index for saving model on the
distributed file system. The server index comes from etcd if etcd is
used, or user can manually specify them. So we need
pserver.NewService() to take index as an argument. Since index could
come from etcd, it would be strange if pserver takes an index as
argument, at the same time get the index from etcd. so we will need to
decouple pserver with etcd.

add eigen as cmake external project