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.

zmq_encoder/decoder are able to add/trim prefixes from messages; fair queueing and load balancing algorithms factorised into separate classes

This reverts commit 88695aae.