Port Executor Manager to Julia

[vchuravy]

For full Module support we will need to port the Python Executor Managers to Julia.

• https://github.com/dmlc/mxnet/blob/master/python/mxnet/executor_manager.py

[pluskid]

The need to support automatic data parallelism seem to be the source that makes a lot of things very complicated (e.g. the existence of an executor manager). I'm wondering maybe we could have a cleaner and easier interface if we give up the auto data-par feature. (Just a random thought)