LU-11297 lnet: MR Routing Feature

LU-11297 lnet: MR Routing Feature

This is a merge commit from the multi-rail branch. It brings in
the MR Routing feature. This feature aligns the LNET Multi-Rail
behavior with routing. A gateway now is viewed as a Multi-Rail
capable node. When a route is added only one entry per gateway
should be used. That route entry should use the primary-nid of
the gateway. The multi-rail selection algorithm is then run when
sending to the gateway to select the best interface to send to.

Furthermore the gateway aliveness is now kept via the health
mechanism. And the gateway pinger now uses discovery instead
of maintaining its own pinger handler.

Signed-off-by: Amir Shehata <ashehata@whamcloud.com>
Change-Id: Ie2d8c6449f84860511b322ff2db3ed656a163e74