LU-9121 lnet: Preferred gateway selection

LU-9121 lnet: Preferred gateway selection

Add mechanism for managing preferred gateway lists.
When selecting a route through a gateway, if there exists
a preferred gateway list for the destination peer, then choose
the preferred gateway. If there are multiple preferred
gateways, to make the selection, use in order of decreasing
importance: route priority, number of hops, number of available
tx credits on the associated lpni and route sequence counters.
If there are no preferred routes, select the best route
available using the same criteria.

Test-Parameters: trivial testlist=lnet-selftest,sanity-lnet
Signed-off-by: Amir Shehata <ashehata@whamcloud.com>
Change-Id: If46920cf7b79aa8b211d6c0a35995edce9b1699a
Reviewed-on: https://review.whamcloud.com/34353
Reviewed-by: Chris Horn <chris.horn@hpe.com>
Reviewed-by: Serguei Smirnov <ssmirnov@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>