Update local C++ simulators to use Qobj internally

[diego-plan9]

What is the expected enhancement?

After #655, the local C++ simulators accept a Qobj instance (with the new schema) during the Python stub's .run(), but internally they still use the old schema (when passing it to the executable during the module's run()). Ideally they should also make use of the new schema and the instances directly instead of performing that conversion.

[ajavadia]

Removed from 0.6 milestone since the c++ simulator is being reworked for Aer. For now Terra can translate new qobj to old qobj.

[jaygambetta]

@chriseclectic this is all done in the basicAer do you see any reason to keep this open.