* add basic interface for learning rate decay
* add exponential_decay
* add natural_exp_decay
* add inverse_time_decay

* Add save_combine_op

* Add load_combine_op and test

* Add unit-test

* Add a delete to free buffer memory

* Add new variant of load/save

* Fix unit-test

* Add another unit test for compatibility with original save/load

* Address review comments and simplify logic

* Address review comments and simplify code - part 2

* Fix naming issues and CMake problems

* Address review comments

* Fix LoD information in tests

* Address review comments: round 2

* Polish sum_op support SelectedRows in_place

* initialize batch barrier

* add some comments

* update

* fix batch barrier

* use sendvariable rpc interface to send batch barrier

* fix comment

* fix method

* fix by comment

* fix by comment

* add Channle

* refine Channel

* Add one_hot operator.

* Add more unit tests.

std::vector<bool> is not safe for concurrent write, even to difference indices.
More discussion:
https://stackoverflow.com/questions/48452611/is-stdfuturewait-a-memory-barrier-i-can-not-explain-this-data-race

* fix call once logic

* clean up

* further clean up

* fix root number

* aysnc send/recv, seriliaze/deserialize using threadpool

* implement paralell deserialization correctly

Unify the recognize_digits