Change the behaviour of thread pool. Thread pool will ignore the
thrown exception implicitly. It is hard to debug.
Corrently, ThreadPool::Run will invoke `LOG(FATAL)` if an exception
thrown.

* refine channel test

* follow comments

* Add dependency enforce to threadpool

* Revert changes to channel_test.cc

* Revert changes to channel_test.cc

* Add #include "paddle/framework/macros.h"

* Polish threadpool

* Add #include <vector>

* Rename variables

* Rename variables

* clang-format

* Run interface return future

* delete unused comments

* "fix threadpool style"

* "remove header"

* implement a simple threadpool

* unlock before cv.notify

* add done function

* add lock with GetAvailable function

* delete done_

* using call_once in GetInstance

* update by comment

* update comment

* enhance unit test for multi threads task