msg/async/Protocol*: send keep alive if existing wins

msg/async/Protocol*: send keep alive if existing wins

This is a follow-up fix of https://github.com/ceph/ceph/pull/25754.

It turns out the existing connection would win the connection race
and then be stuck in the __replacing__ stage forever without being
aware of that the underlying Pipe is actually broken.

Fix by forcing existing sending keepalive periodically __too__.

Fixes: http://tracker.ceph.com/issues/38493
Signed-off-by: xie xingguo <xie.xingguo@zte.com.cn>