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

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

This reverts commit 61b9432ef9a3847eceb96f8d5a854567c49bbf61.

If we are in the middle of replacing, we can not queue any further
write events into the old center because we may end up replacing
existing connection's center with a new one, and hence executing
the newly queued write events in the old thread.

See **transfer_existing** for a detailed description.

Also the patch does not make a lot of sense for the original issue
it tried to resolve, because **send_keepalive** is a pure noop if the
underlying connection is not ready, which is obviously true for the
case demonstrated in http://tracker.ceph.com/issues/38493..

Fixes: http://tracker.ceph.com/issues/38569
Signed-off-by: xie xingguo <xie.xingguo@zte.com.cn>