RDS: Connection handling

While arguably the fact that the underlying transport needs a
connection to convey RDS's datagrame reliably is not important
to rds proper, the transports implemented so far (IB and TCP)
have both been connection-oriented, and so the connection
state machine-related code is in the common rds code.

This patch also includes several work items, to handle connecting,
sending, receiving, and shutdown.

Signed-off-by: Andy Grover <andy.grover@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>