[links] stabilize latency calculation when nodes are not responsive

[links] stabilize latency calculation when nodes are not responsive

The following scenario is more of a corner case than normal, but
this change allows to better deal with this situation:

1) 2 nodes cluster (corosync) (node A and node B)
2) kill -stop $(pidof corosync) on node A
3) node B will continue to send ping packets to node A
4) node A is accumulating those ping packets in the kernel network socket
5) wait some seconds and unpause node A
6) node A will start processing the ping packets in the queue
   and send pong replies to node B
7) node B will see an extreme increase of latency due
   those "obsoleted" ping/pong packets
8) node B, as latency increases, will take longer and longer
   to notice that node A is down due to the pong_timeout adjustment
   for latency (required for initial cluster spike).

the solution:

1) Use average latency to calculate pong_timeout_adj vs latency_max.
   Averate latency will go down again in time, while latency_max is never
   reset.

2) RX thread will filter out all pong packets that have higher latency
   than currently configure pong_timeout. This barrier should have
   been in place even before.

this solution reduces the latency spike on node B to a perfectly
reasonable level and it will all eventually stabilize over time
as latency samples increase and latency will reduce.

Please be aware that using a pong_timeout smaller than latency will
simply mark the link down now.

Signed-off-by: Fabio M. Di Nitto <fdinitto@redhat.com>