Removal of ZMQ_IDENTITY resulted in various session classes doing
almost the same thing. This patch merges the classes into a single
class.
Signed-off-by: Martin Sustrik <sustrik@250bpm.com>

Signed-off-by: Martin Sustrik <sustrik@250bpm.com>

Signed-off-by: Martin Sustrik <sustrik@250bpm.com>

Signed-off-by: Martin Sustrik <sustrik@250bpm.com>

Signed-off-by: Dhammika Pathirana <dhammika@gmail.com>

Signed-off-by: Martin Sustrik <sustrik@250bpm.com>

This commit introduces the necessary changes necessary
for implementing flow control. None of the socket types
implements the flow control yet. The code will crash when
the flow control is enabled and the thw lwm is reached.

The following commits will add flow-control support for
individual socket types.