Made replay-on-join more enjoyable
Made replay-on-join more enjoyable

People have been complaining that upon joining a Teleport session they
do not always see anything.

While teleport instanly replays the most recent chunk, the chunk itself
could be anything: backspace, space, enter, etc. So often people are
greeted with a blank screen.

This commit replays the last 20 writes, which always gvies a newly
joined party a nice perspective on what's been going on.

Also, fixed unlikely-to-happen but still real race condition.

sokoow/teleport,ajvb/teleport,ajvb/teleport,gravitational/teleport,sokoow/teleport,suedadam/teleport,yacloud-io/teleport,ajvb/teleport,yacloud-io/teleport,gravitational/teleport,suedadam/teleport,yacloud-io/teleport,suedadam/teleport,sokoow/teleport,tidalf/teleport,sokoow/teleport,yacloud-io/teleport,tidalf/teleport,tidalf/teleport,gravitational/teleport,sokoow/teleport,bshelton229/teleport,tidalf/teleport,suedadam/teleport,ajvb/teleport,bshelton229/teleport,bshelton229/teleport,gravitational/teleport,aashley/teleport,ajvb/teleport,aashley/teleport,tidalf/teleport,gravitational/teleport,aashley/teleport,bshelton229/teleport,gravitational/teleport,aashley/teleport,aashley/teleport,yacloud-io/teleport,suedadam/teleport,bshelton229/teleport,sokoow/teleport,yacloud-io/teleport,yacloud-io/teleport