vpnkit-forwarder: avoid writing 2 bytes at a time
vpnkit-forwarder: avoid writing 2 bytes at a time

This writes via `*bufio.Writer` with an explicit `Flush` after sending
a frame or data payload. The original `net.Conn` taken by `NewMultiplexer`
can now be stored downgraded as an `io.Closer` as all I/O is done via the
separate reader and writer.

Signed-off-by: David Scott <63c9eb0ea83039690fefa11afe17873ba8278a56@docker.com>

djs55/vpnkit,djs55/vpnkit,djs55/vpnkit