Unset unittests http_proxy env to avoid timeout.

Polish fleet API to support cuda collective mode and nccl2 mode

* add is_runnning in communicator, test=develop

* optimize communicator merge sparse gradient test=develop

* revert multithread selected rows merge add test=develop

* follow comment test=develop

* fix save/load in Fleet
* add UT framework of Fleet

* remove attribute in Allocator::Allocate, test=develop

* fix travis ci error, test=develop

Async exe support communicator

test=develop

* optimize communicator flag

* change flags in init py test=develop

test=develop