LU-12941 lnet: Add peer level aliveness information

LU-12941 lnet: Add peer level aliveness information

Keep track of the aliveness of a peer so that we can optimize for
situations where an LNet router hasn't responded to a ping. In
this situation we consider all routes down, and we needn't spend time
inspecting each route, or inspecting all of the router's local and
remote interfaces in order to determine the router's aliveness.

Cray-bug-id: LUS-7860
Test-Parameters: trivial
Signed-off-by: Chris Horn <hornc@cray.com>
Change-Id: Ie63c1ef40de3ad818639bae6b040923898fd5b46
Reviewed-on: https://review.whamcloud.com/36678
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Neil Brown <neilb@suse.de>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Alexey Lyashkov <c17817@cray.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>