GENERIC allows to use 0MQ as a dumb networking framework.
It provides user with connect/disconnect notifications.
Also, each inbound message is labeled by ID of the connection
it originated from. Outbound messages should be labeled by
the ID of the connection to send them to.

To distinguish connect/disconnect notifications from common
messages, COMMAND flag was introduced.
Signed-off-by: Martin Sustrik <sustrik@250bpm.com>