net/conn: io, not channels

This commit changes the connections to use io.ReadWriters
instead of channels (+ async workers). This is a pretty
big change -- away from csp -- in the name of performance
(and predictable flow control).

It also uses the brand new secio, which is spipe's successor.
multiconn: close fanIn + error
added ctxgroup and router
updated msgio (bufixes)
new swarm with io and router
muxer io
stash
updated msgio + secio
make net/conn.Conn conform to net.Conn
multierr
much simpler net

- removed ctxcloser
- removed multiconn
- focused on netio
new swarm -- it's so simple
conn: tests pass :)
added loggables pkg
writing swarm2 using go-peerstream

omg wow such pass
handler muxer
refactor SwarmConn -> Conn
swarm fixes
moved mux into net
make net work with new stream + mux