rsocket: Add datagram support

rsocket: Add datagram support

Add datagram support through the rsocket API.

Datagram support is handled through an entirely different protocol and
internal implementation than streaming sockets.  Unlike connected rsockets,
datagram rsockets are not necessarily bound to a network (IP) address.
A datagram socket may use any number of network (IP) addresses, including
those which map to different RDMA devices.  As a result, a single datagram
rsocket must support using multiple RDMA devices and ports, and a datagram
rsocket references a single UDP socket, plus zero or more UD QPs.

Rsockets uses headers inserted before user data sent over UDP sockets to
resolve remote UD QP numbers.  When a user first attempts to send a datagram
to a remote address (IP and UDP port), rsockets will take the following steps:

1. Store the destination address into a lookup table.
2. Resolve which local network address should be used when sending
   to the specified destination.
3. Allocate a UD QP on the RDMA device associated with the local address.
4. Send the user's datagram to the remote UDP socket.

A header is inserted before the user's datagram.  The header specifies the
UD QP number associated with the local network address (IP and UDP port) of
the send.

A service thread is used to process messages received on the UDP socket.  This
thread updates the rsocket lookup tables with the remote QPN and path record
data.  The service thread forwards data received on the UDP socket to an
rsocket QP.  After the remote QPN and path records have been resolved, datagram
communication between two nodes are done over the UD QP.

Signed-off-by: Sean Hefty <sean.hefty@intel.com>