Add convert to recordio function

Set ps_desired when pserver init

Add paddle/memory/README.md

Add recordio dependency

Move directory third_party into /build

Adding platform/must_check.h

__must_check is a macro mark of function return value. It let developer
must check the return value is legal or not.

Implement framework::Variable

Fix macos compile

Please use `override` not `virtual` in sub-classes.
`override` can check if there is a method in `parent` while compiling.

Master save and load state from etcd, and register on etcd.