librdkafka C++ interface

See rdkafkacpp.h for the public C++ API

Maintainer notes for the C++ interface:

• The public C++ interface (rdkafkacpp.h) does not include the public C interface (rdkafka.h) in any way, this means that all constants, flags, etc, must be kept in sync manually between the two header files. A regression test should be implemented that checks this is true.

• The public C++ interface is provided using pure virtual abstract classes.