[zmq] Distinguish Round-Robin/Fanout socket sending mode

For zmq.DEALER socket there are two round-robin modes exist,
controlled by zmq.IMMEDIATE option. Immediate True means sending
only to running servers and False means to send to all connected
servers (may be not up and running at the moment).

If we do all messaging over DEALER socket as we do for direct
connections, immediate=True better suits for direct CALL/CAST
message types and immediate=False is better for fanout.

This patch fixes things for dynamic connections as for static
we already used the approach.

Change-Id: I70c7aeb3c7a2c63c128bec394827577cab580def