socket: restructure disconnect/poll-thread interactions

Change-Id: I792c28f52068e4ed666069b740739662685160bc
BUG: 906401
Signed-off-by: Jeff Darcy <jdarcy@redhat.com>
Reviewed-on: http://review.gluster.org/4456
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Anand Avati <avati@redhat.com>