LU-11378 lnet: MR aware gateway selection

LU-11378 lnet: MR aware gateway selection

When selecting a route use the Multi-Rail Selection algorithm to
select the best available peer_ni of the best route. The selected
peer_ni can then be used to send the message or to discover it
if the gateway peer needs discovering.

Test-Parameters: forbuildonly
Signed-off-by: Amir Shehata <ashehata@whamcloud.com>
Change-Id: I376af57611591eed2eb1edb80a1b3a68b5aefd19
Reviewed-on: https://review.whamcloud.com/33188
Reviewed-by: Olaf Weber <olaf.weber@hpe.com>
Tested-by: Jenkins