go: mux: when dispatcher shuts down, close channels
go: mux: when dispatcher shuts down, close channels

Previously open channels would remain open and reads would block
forever because no data or close messages could arrive. Instead
we should close any open channels when the dispatcher shuts down.

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

djs55/vpnkit,djs55/vpnkit,djs55/vpnkit