rx: allow links to be active faster

scenario: node A and node B

node A completes ping/pong link up process faster than B
node A starts to send data to node B

old behavior: node B would unconditionally discard data packets

new behavior:
node B will check internal link status to see if ping/pong link up
process is in progress.

if the link is sending pings and receiving pongs, it´s safe to assume
that the link will go up (since we are also receiving data from the
node A). node B will speed up link up and accept data immediately.

This should address a overall init race condition when nodes startup
are slighly off by enough milliseconds to cause 2 memeberships to form
and see fencing fireworks.

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