Callbacks are called when the data buffer of the message assiciated
with the corresponding region is no longer needed by the transport.
Example in examples/advanced/Region/
* FairMQTransportFactoryZMQ: move the config invariant
initialization to ctor
* FairMQChannel: add new ctor that creates usable channel
* FairMQSocket*: close sockets in dtor
* FairMQTransportFactory*: terminate context in dtor
* FairMQChannel: add Bind/Connect facades (for explicit control, e.g. timing)
- give transport Initialize() method with access to device config.
- avoid using global context in the transport.
- simplify shutdown procedure (no need for extra thread).
- Avoid polling when only one input channel is used.
- Send only handles for shared memory transport.
- Avoid waiting in the rate logger thread when nothing to log.
- Hide warnings from generated files
- Fix#483
