Problem: need a client stack

Solution: build proper client stack, at least for stream class.

This uses the zproto_client generator. It's fully async and runs as
a CZMQ actor, handling heartbeats and connection expiry. The API is
in include/mlm_client.h.
Problem: need functional main server

Solution: start/stop mlm_server actor from malamute.c
Problem: need a command-line shell

Solution: write a command-line shell, 'mshell', that implements a
trivial reader and writer for command line usage.